Initialize reference count for SnapshotTracker

Because there can be more than one onTaskSnapshotChanged listeners in
a process, initialize the reference count based on the listener size
for SnapshotTracker, to prevent one thread closing the snapshot while
others are still using it.
Also remove the extra close buffer request when
TaskStackChangeListeners#impl handle ON_TASK_SNAPSHOT_CHANGED,
ThumbnailData#fromSnapshot will do close buffer already.

Flag: com.android.window.flags.reduce_task_snapshot_memory_usage
Bug: 438037933
Test: monitor the same snapshot object will only request to close
buffer once from other tests after SystemUITests:NavigationBarTest

Change-Id: I31e94bf377b4dab998691ee962172dcee082b956