Fix graphical glitches on keyguard-unlock with PIP

Since keyguard animations sync transactions to view rendering,
it's possible for them to apply changes after the finishtransaction,
so wrap containers in leashes so that we can just detach them.

Also, remove the single-task-opening restriction now that keyguard
animations support multiple tasks.

Bug: 278886491
Test: app in pip, turn screen off, turn screen on+unlock.
Change-Id: Iea2fc2d5ae16c59e7983b87fea2cd62d327f4ac9