Fix crash in ChooseLockPassworkFragment.

Check for null activity when the handler receives messages to ensure
that the fragment is still attached to the activity.

Fixes: 31034901

Test: Manual - Manually try to update password, verify it works. Run
monkey test on settings package and verify no crash seen.

Change-Id: I9753729ee64458fa1bc25d78abc946d2563a5067
1 file changed