Move startInput for WINDOW_FOCUS_GAIN to background thread

For a typical window with Editor, IMM#startInputOrWindowGainedFocus
is called twice: first for WINDOW_FOCUS_GAIN (dummyInput), then
CHECK_FOCUS when actual editor receives focus and startsInput.
The first once can be moved to background thread and second startInput
will wait till its finished.

Bug: 139806621
Test: Manually launch activities with editors and confirm no change in
behavior.

Change-Id: I6aa4a664cfd0c86f75cee2457715317194bbe5e2