Add new setInputMethodEnabled API

Make accessibility services able to enable/disable a specified
IME in the same package for the current user.

Bug: 195476910
Fix: 191386474

Test: atest AccessibilitySoftKeyboardTest. Also tested with
modified talkback (use the swith input method API for enable
ime), and self created empty a11y service with an empty ime.
Mananged device and profile which set limit on user 0 are also
tested.

Change-Id: I4187468076705ac597d680f2f5dc32d7b166da1f