Use WeakReference in RemoteFillService.

This mitigates the issue of lingering IFillCallback and ISaveCallback
stub allocations. After the change, Binder would only hold onto the new
thin callback wrappers. The actual underlying stubs would not be
prevented from being GC'ed.

Bug: 307972253
Test: atest CtsAutoFillServiceTestCases
Change-Id: I7860ad66be134a5328e8dc88e71e665406d51d7d