InputManager: Add system setting to define keyboard locales

KEYLAYOUT_OVERRIDES expects a string of "devicename,layoutfile" to set
keyboard layout and character map to files in /system/usr/keylayout and
/system/user/keychars.

This overrides the default behaviour of opening files that match the
device's input name if it is unconfigured.

Change-Id: I35d07c6986db5a8b0744ff4c8649c2a34cc2fd14