Set isInPictureInPictureMode earlier

Per https://developer.android.com/guide/topics/ui/picture-in-picture
After app requests enterPictureInPictureMode and receives onPause
callback, it will continue playback if isInPictureInPictureMode is true.

However, with ag/11273366, isInPictureInPictureMode will now return true
after the new configuration is dispatched to app, which happens after onPause.
This may cause app, following the guidance, to cease playback in PiP mode.

Fixes this by setting the internal mIsInPictureInPictureMode earlier
right in enterPictureInPictureMode

Video: http://go/recall/-/aaaaaabFQoRHlzixHdtY/fVRqG7UWoKkQQhFxPkzcUt
Bug: 156924033
Test: manually enter PiP from Twitch
Change-Id: I8e0865076fcb756cfa5db39901f460ab5ad69b99

core/java/android/app/Activity.java

@@ -2838,7 +2838,13 @@ public class Activity extends ContextThemeWrapper
                 throw new IllegalStateException("Activity must be resumed to enter"
                         + " picture-in-picture");
             }
-            return ActivityTaskManager.getService().enterPictureInPictureMode(mToken, params);
+            // Set mIsInPictureInPictureMode earlier and don't wait for
+            // onPictureInPictureModeChanged callback here. This is to ensure that
+            // isInPictureInPictureMode returns true in the following onPause callback.
+            // See https://developer.android.com/guide/topics/ui/picture-in-picture for guidance.
+            mIsInPictureInPictureMode = ActivityTaskManager.getService().enterPictureInPictureMode(
+                    mToken, params);
+            return mIsInPictureInPictureMode;
         } catch (RemoteException e) {
             return false;
         }