Use WeakReference for the TaskSnapshotWindow in inner Window.

Use WeakReference for the TaskSnapshotWindow so it can be clear at GC.
Also clear the TaskSnapshotWindow reference in
mStartingWindowRecords since it should be removed immediately.

Still observer the native Binder reference could hold the reference to
the TaskSnapshotWindow$Window object, from the local test it can be
released after force GC(need to trigger on system_server and systemUI).
So at least with this patch, TaskSnapshotWindow object can be released
immediately.

Bug: 225363815
Test: open several tasksnapshot starting window, verify the memory can
be released on perfetto.
Test: atest TaskSnapshotWindowTest StartingSurfaceDrawerTests

Change-Id: Iaa0a266c2c6756cef24d92cde57a6debc947e133