[Memory Leak] Avoid false-positive leak detection of TaskbarActivityContext...

[Memory Leak] Avoid false-positive leak detection of TaskbarActivityContext via RotationButtonController's IRotationWatcher.Stub

Binder obj's lifecycle depends on remote process's GC and there is no
way for leak canary to coordinate with remote process's GC to watch
obj's lifecycle. Thus we explicitly clear the ref to host obj when
unregister IRotationWatcher.Stub.

In addition, user can rotate screen frequently and accumulate a few
IRotationWatcher.Stub and TaskbarActivityContext. These obj awaits
remote process's GC to be GCed from Launcher. This CL allows GC
these expensive objs earlier and should lower HWM memory usage.

Fix: 434032994
Flag: NONE - released code
Test: manual
Change-Id: Iadb6e0af4a63117294db5cb099ec06cfaab40761