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().

This also fixes an issue with the AE case which is why
the earlier version of this CL was reverted at ag/28822947

Bug: 359944953
Flag: com.android.wm.shell.enable_pip2_implementation
Test: atest PinnedStackTests#testAutoEnterPictureInPictureLaunchActivity
Test: atest SplitAttributesCalculatorTest
Change-Id: I30a28ad60a01d3f2192644ef150b22f7f36f7220