Modify the case in adjustFocus when the mFocusedActivity is null

Starting activity failed and the mFocusedActvity become null,
consider this condition when adjust focus.

Because in this condition, the focus mismatch between
ActivityManagerService#mFocusedActivity and
InputDispatcher(inputfilnger)#mFocusedApplication, ANR may occur.

Test: monkey test 48h with no ANR

Change-Id: Id1a2bf0d6ca6530e34443d359fbe2aa6c11b878c