Add a listener when task snapshots change

Since we start recents before we take the snapshot, we need to add
a mechanism to inform recents about task snapshots changes.

We add a new method to TaskStackChangedListener,
onTaskSnapshotChanged, which gets called whenever a task snapshot
changes. Then, SystemUI registers such a listener and updates the
task thumbnail view for the specific task.

Test: Open app, press recents, make sure thumbnail is up-to-date
Bug: 31339431
Change-Id: I01e81b9cd11886da734da671c68d5732aa51009f