Dispatch KEYCODE_BACK to the full input pipeline in compat callback.

Also fix an issue that back callback is invoked both on ACTION_DOWN and
ACTION_UP.

Bug: 222408115
Fixes: 227161441
Fixes: 222408115
Fixes: 226301000
Test: Bring up IME from an EditText (e.g. Settings search bar), try
swipe back, make sure IME is dismissed and not the activity.

Change-Id: I7e5eeab276f70665d1fc37a0ef333f506fe305ec