Alert shows there are cases of ~400 lingering session objects held by RemoteFillService.

RemoteFillService references Session by mCallbacks variable (Session
implements FillServiceCallbacks interface). This CL sets reference to
mCallbacks to null when doing unbind(), and hope to reduce lingering session objects after its
due lifetime.

Bug: b/287677604
Test: atest CtsAutoFillServiceTestCases

Change-Id: I65de76ae6d8e88642658b73283a41c53fcc5d610