Bug 936723 - [keyboard] don't allow user to enable keyboard apps that
don't have required trust level.
 - Filter out keyboard apps that do not have input permission and
   correct privilege level.
Merge pull request #13693 from RudyLu/keyboard/Bug936723-filter_unprivileged_keyboard_apps

Bug 936723 - [keyboard] don't allow user to enable keyboard apps that
r=gnarf, gary