LatinIME: Fix back-arrow finishing activity

* When going to a sub setting menu and pressing the
  action bar up arrow instead of the device's back
  key, the settings are closed instead of going
  one level up as expected
* Call "onBackPressed" instead of "finish" to fix
  that
* For InputMethodSettings create a new flag to
  alter the behaviour for our use case but to keep
  existing behaviour for other callers

Test:
- Open the keyboard settings from Settings app
and navigate between menus ->
action bar back icon only goes one levelup
- Open keyboard settings directly from keyboard:
behaviour is the same, last "up" action takes you
back to where you came from

Change-Id: I4f01e80e7f35ddbd2af1baae899d689f1144a5a1