Make mCurrentPipTaskToken null when app crashes

PipTransition#mCurrentPipTaskToken needs to be nullified
after exit. Otherwise, we end up calling resetPrevPip()
without startEnterAnimation() later on when running PinnedStackTests.

Bug: 287989186
Test: atest PinnedStackTests
Test: run "adb shell am force-stop <pip activity package name>"
	after entering PiP, and then check for
	mCurrentPipTaskToken in sysdump
Change-Id: Id869f78dbf85f009509082181590e224a6d7f893