FocusRequester: fix NPE

Fix NPE and potential race condition in
FocusRequester.dispatchFocusResultFromExtPolicy where mFocusDispatcher
was checked against null, but the method didn't return in that case.

Bug: 142278598
Test: Android Auto with phone call and custom focus policy
Change-Id: I88447a255132c94f8c2fbf97a1e3ee88f6d2e993