Check if activity entering PiP is a top activity

We need to make sure we handle any trampoline activity
start cases where, for instance, some Activity A,
start another activity B on creation, but then,
the activity A calls to enterPictureInPictureMode().

Bug: 359944953
Flag: com.android.wm.shell.enable_pip2_implementation
Test: atest PinnedStackTests#testAutoEnterPictureInPictureLaunchActivity
Change-Id: Ic3e7db08c8b552e2d83d6edac00035b249e3af85