Fix back gestrure doesn't work and ANR happens

The desired focused window may become unfocusable when user swiping
task to recents or in exit animating. We need to clear the current
focus when it can't be focusable so it could set the focus again
when the focus is back to the task.

Test: atest WindowFocusTests
Test: swiping task to recents, hold and back to task, back gesture.
Bug: 173380433
Change-Id: I3cb341735e0711e6ede45c429b30328fcae03670