Allow enabled IMEs to be disabled in Settings

* If an IME is not in the permitted IME list and is enabled,
  the user should be able to disable it.
* This prevents the IME from being stuck in a state where the
  user cannot disable it and as a result the user's device
  becomes uncompliant with their IT admin's policy.

Manual testing steps:
* Install TestDPC
* Set up work profile using TestDPC
* Install swiftkey in the work profile
* set permitted input methods (in TestDPC) to only system
* Use adb to enable Swiftkey
* Go to Settings and verify Swiftkey can be disabled
* Once disabled, verify Swiftkey cannot be re-enabled.

Bug: 180517539
Test: manual testing
Change-Id: I7bdd3a0c4fed4d16006445b7774fd1006cc2b569