Do not merge PiP removal into Recents

After entering PiP from a multi-Activity Task and then remove the
original Task from Overview, the TO_BACK transition that dismiss the PiP
task will be issued and merged into the Recents transition. In turn, it
won't be played till the Recents transition is finished, e.g. by user
tapping to return home or etc.

This in general results no issue since the end state (after return home)
is correct. It does cause visual artifact, like empty PiP window, for
some apps though if they choose to stop rendering if the original Task
is dismissed.

Flag: EXEMPT bugfix
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/cRzOITGISz2SulG0c5Qgj8
Bug: 370682472
Test: atest PinnedStackTests
Test: manual, see the video
Change-Id: I864e9cb0e8f04f9b692072af73d4dd660962bc65