Tracking task snapshot usage in client process.(4/N)

- Introduce ITaskSnapshotListener, separate task snapshot related
callbacks from TaskStackListener.
- Prepare to deprecate onTaskSnapshotChanged and
onTaskSnapshotInvalidated in TaskStackListener.
- Relocate some task snapshot access APIs to TaskSnapshotManager.
- Add a reference count for the tracker when returning an existing
TaskSnapshot object from the global tracker.

Flag: com.android.window.flags.reduce_task_snapshot_memory_usage
Bug: 238206323
Test: Use "dumpsys meminfo" to check how many snapshots remain in a
process, and where they are used.

Change-Id: I122898d99e084297a7bea20695d2720fc05533be