Fix crash when loading PDF from cloud storage via SAF

[PatrykMis]

Let's say we have PDF on Nextcloud which can be accessed with SAF. File is not downloaded offline before. When trying to open via AOSP file manager or PDFViewer:

type: crash
osVersion: google/shiba/shiba:14/AP2A.240905.003/2024092900:user/release-keys
package: app.grapheneos.pdfviewer:19
process: app.grapheneos.pdfviewer
processUptime: 378 + 205 ms
installer: app.grapheneos.apps

java.lang.RuntimeException: Unable to start activity ComponentInfo{app.grapheneos.pdfviewer/app.grapheneos.pdfviewer.PdfViewer}: android.os.NetworkOnMainThreadException
	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:4004)
	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4184)
	at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:114)
	at android.app.servertransaction.TransactionExecutor.executeNonLifecycleItem(TransactionExecutor.java:231)
	at android.app.servertransaction.TransactionExecutor.executeTransactionItems(TransactionExecutor.java:152)
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:93)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2602)
	at android.os.Handler.dispatchMessage(Handler.java:107)
	at android.os.Looper.loopOnce(Looper.java:232)
	at android.os.Looper.loop(Looper.java:317)
	at android.app.ActivityThread.main(ActivityThread.java:8623)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
	at com.android.internal.os.ExecInit.main(ExecInit.java:50)
	at com.android.internal.os.RuntimeInit.nativeFinishInit(Native Method)
	at com.android.internal.os.RuntimeInit.main(RuntimeInit.java:369)
Caused by: android.os.NetworkOnMainThreadException
	at android.os.Parcel.createExceptionOrNull(Parcel.java:3202)
	at android.os.Parcel.createException(Parcel.java:3176)
	at android.os.Parcel.readException(Parcel.java:3152)
	at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:202)
	at android.database.DatabaseUtils.readExceptionWithFileNotFoundExceptionFromParcel(DatabaseUtils.java:165)
	at android.content.ContentProviderProxy.openTypedAssetFile(ContentProviderNative.java:814)
	at android.content.ContentResolver.openTypedAssetFileDescriptor(ContentResolver.java:2068)
	at android.content.ContentResolver.openAssetFileDescriptor(ContentResolver.java:1883)
	at android.content.ContentResolver.openInputStream(ContentResolver.java:1553)
	at app.grapheneos.pdfviewer.PdfViewer.z(Unknown Source:14)
	at app.grapheneos.pdfviewer.PdfViewer.onCreate(Unknown Source:693)
	at android.app.Activity.performCreate(Activity.java:9013)
	at android.app.Activity.performCreate(Activity.java:8991)
	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1531)
	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3986)
	... 15 more

If the only way to access that kind of file is to grant network permission, the user should be notified the file have to be downloaded first (either by making it offline via Nextcloud app or moving it to device's local storage first).