Make IMS.setBackDisposition() work.

InputMethodService.setBackDisposition() has been broken from a long
time. This is how setBackDisposition() is supposed to work
1. BACK_DISPOSITION_WILL_DISMISS:
When Keyboard is visible back button will show as down arrow (in
navigation bar) and tapping it will dismiss keyboard.
2. BACK_DISPOSITION_WILL_NOT_DISMISS:
When keyboard is visible, back button will be shown as BACK arrow and
tapping it will send the back event to current activity. If activity
doesn't handle onKeyDown() for back button, activity will finish and
keyboard will hide.

Note: backDisposition flags reset when finishInput() is called.

Bug: 38513361
Test: Manual with apk attached in bug
Test: atest InputMethodServiceTest
Change-Id: I99e7c413fe1a93f8d8cff897b5c5f0947690d2c0