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
(cherry picked from commit 755d0c3df201f920634a473ae94b7eb1f0cad0a3)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:931efa54c0a0d2419c68787a31f73dc5013d7b16)
Merged-In: Id869f78dbf85f009509082181590e224a6d7f893
Change-Id: Id869f78dbf85f009509082181590e224a6d7f893