Choose opaque activity for fixed rotation with entering pip

The case is like: (A,B,C are activities from top to bottom)
A orientation=unspecified, dialog style(!fillsParent)
B orientation=landscape, entering pip mode
C orientation=portrait, the activity that decides screen orientation

The non-opaque activity won't decide screen orientation, that causes
to miss the proper timing to trigger fixed rotation when the previous
top is switching from fullscreen to pip with screen orientation change.

This makes the callback order of PipTaskOrganizer still follows first
onFixedRotationStarted and then onTaskAppeared. So the PiP won't stay
in a stale state with wrong bounds or alpha.

Bug: 204844178
Test: 1. Launch Chrome and play video in fullscreen landscape.
      2. Enable auto-rotation, put device in landscape.
      3. adb shell am start -a android.intent.action.VIEW
      Chrome should enter PiP correctly.
Change-Id: I34250bb879898059568bc3b3b6b4da88fea75f57