Fixing issue with activity incorrectly auto-entering PiP.

- When the PiP menu activity was brought forward, we were inadvertently
  setting the auto-enter supported flag for the PiP base activity as
  well, which would trigger auto-enter PiP once the activity is moved
  back to fullscreen and then subsequently finished.

Bug: 36352343
Bug: 36364010
Test: android.server.cts.ActivityManagerPinnedStackTests
Change-Id: Id1b800bf392d03b55562fb13374e33a5ff0ebda6