Fix context leak in AudioSwitchPreferenceController.

In this controller the context is stored into a global singleton but was
not cleared, thus leaking context and associated views.

Change-Id: I4247f8ff753bc0a331c6c81a0e4b5b4bc41588de
Fixes: 80507279
Test: robotests
Test: inspected hprof before/after change
2 files changed