[MDCKeyboardWatcher] Migrate off of deprecated mainScreen usage.

PiperOrigin-RevId: 596024734

components/private/KeyboardWatcher/src/MDCKeyboardWatcher.m

@@ -16,7 +16,6 @@
 
 #import <CoreGraphics/CoreGraphics.h>
 
-#import "MDCAvailability.h"
 #import "MaterialApplication.h"
 
 NS_ASSUME_NONNULL_BEGIN
@@ -95,44 +94,18 @@ - (void)updateKeyboardOffsetWithKeyboardUserInfo:(NSDictionary *)userInfo {
     return;
   }
 
-  UIWindow *keyWindow = nil;
-#if MDC_AVAILABLE_SDK_IOS(15_0)
-  if (@available(ios 15.0, *)) {
-    for (UIScene *scene in [UIApplication mdc_safeSharedApplication].connectedScenes) {
-      if ([scene isKindOfClass:[UIWindowScene class]] &&
-          scene.activationState == UISceneActivationStateForegroundActive) {
-        keyWindow = ((UIWindowScene *)scene).keyWindow;
-        break;
-      }
-    }
-  } else {
-    for (UIWindow *window in [UIApplication mdc_safeSharedApplication].windows) {
-      if (window.isKeyWindow) {
-        keyWindow = window;
-        break;
-      }
-    }
-  }
-#else
-  for (UIWindow *window in [UIApplication mdc_safeSharedApplication].windows) {
-    if (window.isKeyWindow) {
-      keyWindow = window;
-      break;
-    }
-  }
-#endif
-  CGRect screenBounds = keyWindow.screen.bounds;
-  // Keyboard occupies full-width of the screen under both single scene and multiple scenes.
+  CGRect keyWindowBounds = [UIApplication mdc_safeSharedApplication].keyWindow.bounds;
+  CGRect screenBounds = [[UIScreen mainScreen] bounds];
   CGRect intersection = CGRectIntersection(screenBounds, keyboardRect);
 
   // If the extent of the keyboard is at or below the bottom of the screen it is docked.
   // This handles the case of an external keyboard on iOS8+ where the entire frame of the keyboard
   // view is used, but on the top, the input accessory section is show.
-  BOOL dockedKeyboard = CGRectGetMaxY(keyWindow.bounds) <= CGRectGetMaxY(keyboardRect);
+  BOOL dockedKeyboard = CGRectGetMaxY(keyWindowBounds) <= CGRectGetMaxY(keyboardRect);
 
   // If the bottom of the keyboard isn't at the bottom of the screen, then it is undocked, and we
   // shouldn't try to account for it.
-  if (keyWindow != nil && dockedKeyboard && !CGRectIsEmpty(intersection)) {
+  if (dockedKeyboard && !CGRectIsEmpty(intersection)) {
     self.keyboardFrame = intersection;
   } else {
     self.keyboardFrame = CGRectZero;