Fix update wrong new pending transition finishcallback

Sometimes if last transition completing, but another splitscreen
transitions again, the current finishCallback will changed, it will
lead to the last transition cant finish.

Bug: 338007239
Test: manual test apk split enter and dismiss quickly
(cherry picked from https://android-review.googlesource.com/q/commit:8c078a492345dada4817bafb615843e2f25779a2)
(cherry picked from https://android-review.googlesource.com/q/commit:f3895b3daf481b1337837d9e0f2818ba3bc8b625)
Merged-In: I226fc026890459a37d72fc4d833543fae1f09db7
Change-Id: I226fc026890459a37d72fc4d833543fae1f09db7