switch to using Fragments and ViewModel

- Host the WebView and all of its related code inside of a Fragment
instead of an Activity. Fragments can give us more flexibility in the
future on how we can display the WebView, how we could manage some
bottom menu via a BottomNavigationView or similar, etc.

- Use a ViewModel and LiveData architecture in order to move away from
Loaders, which are now deprecated. Almost all state information for the
viewer (like current page, zoom ratio) is now stored in the ViewModel,
which survives configuration changes.
  - Rewrite the document property parsing in DocumentPropertiesLoader in
    Kotlin, taking advantage of Kotlin coroutines to do asynchronous
    parsing of the document properties.

- Dynamically update the properties dialog after PDF loads.
If viewing the PDF properties while the PDF loads, before, the dialog
would show an error message. Now, the error message will be swapped out
with the parsed info as soon the document properties have been parsed.
This is done using an Observer on LiveData.

- Use an alpha version of AndroidX Fragment so that we can use a simpler
way to pass data between two Fragments:
https://developer.android.com/training/basics/fragments/pass-data-between

- Alpha version of Fragments also has ActivityResultLauncher, which
simplifies the SAF launch
https://developer.android.com/training/basics/intents/result
"While the underlying startActivityForResult() and onActivityResult()
APIs are available on the Activity class on all API levels, it is
strongly recommended to use the Activity Result APIs introduced in
AndroidX Activity 1.2.0-alpha02 and Fragment 1.3.0-alpha02."

Closes GrapheneOS#69, GrapheneOS#70

app/build.gradle

@@ -1,4 +1,5 @@
 apply plugin: 'com.android.application'
+apply plugin: 'kotlin-android'
 
 android {
     compileSdkVersion 29
@@ -36,6 +37,12 @@ dependencies {
     implementation fileTree(include: ['*.jar'], dir: 'libs')
     implementation 'androidx.appcompat:appcompat:1.1.0'
     implementation 'com.google.android.material:material:1.1.0'
+    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
+    implementation "androidx.core:core-ktx:1.3.1"
+    implementation "androidx.fragment:fragment:1.3.0-alpha08"
+    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0"
+    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.9"
 }
 
 def props = new Properties()
@@ -61,3 +68,6 @@ if (propFile.canRead()) {
     println 'signing.properties not found'
     android.buildTypes.release.signingConfig = null
 }
+repositories {
+    mavenCentral()
+}

app/src/debug/res/values/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="app_name">PDF Viewer Debug</string>
+</resources>

app/src/main/AndroidManifest.xml

@@ -1,29 +1,38 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-        package="org.grapheneos.pdfviewer"
-        android:targetSandboxVersion="2">
-    <application android:icon="@mipmap/ic_launcher"
-            android:roundIcon="@mipmap/ic_launcher_round"
-            android:label="@string/app_name"
-            android:theme="@style/AppTheme"
-            android:allowBackup="true">
-        <activity android:name=".PdfViewer"
-                android:label="@string/app_name">
+    package="org.grapheneos.pdfviewer"
+    android:targetSandboxVersion="2">
+
+    <application
+        android:allowBackup="true"
+        android:icon="@mipmap/ic_launcher"
+        android:label="@string/app_name"
+        android:roundIcon="@mipmap/ic_launcher_round"
+        android:theme="@style/AppTheme">
+        <activity
+            android:name=".PdfViewerActivity"
+            android:label="@string/app_name">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
+
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
             <intent-filter>
                 <action android:name="android.intent.action.VIEW" />
+
                 <category android:name="android.intent.category.DEFAULT" />
                 <category android:name="android.intent.category.BROWSABLE" />
+
                 <data android:mimeType="application/pdf" />
             </intent-filter>
         </activity>
 
-        <meta-data android:name="android.webkit.WebView.MetricsOptOut"
-                android:value="true" />
-        <meta-data android:name="android.webkit.WebView.EnableSafeBrowsing"
-                android:value="false" />
+        <meta-data
+            android:name="android.webkit.WebView.MetricsOptOut"
+            android:value="true" />
+        <meta-data
+            android:name="android.webkit.WebView.EnableSafeBrowsing"
+            android:value="false" />
     </application>
-</manifest>
+
+</manifest>

app/src/main/java/org/grapheneos/pdfviewer/PdfViewerActivity.java

@@ -0,0 +1,24 @@
+package org.grapheneos.pdfviewer;
+
+import android.os.Bundle;
+
+import androidx.appcompat.app.AppCompatActivity;
+
+public class PdfViewerActivity extends AppCompatActivity {
+
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        setContentView(R.layout.activity_pdf_viewer);
+
+        if (savedInstanceState == null) {
+            PdfViewerFragment fragment = PdfViewerFragment.newInstance();
+            getSupportFragmentManager()
+                    .beginTransaction()
+                    .setPrimaryNavigationFragment(fragment)
+                    .add(R.id.pdf_fragment_container, fragment)
+                    .commit();
+        }
+
+    }
+}