Add new disabled state for "optional" built-in apps.

Add new disabled state for "optional" built-in apps.

The disabled state allows you to make an app disabled
except for whatever parts of the system still want to
provide access to them and automatically enable them
if the user want to use it.

Currently the input method manager service is the only
part of the system that supports this, so you can put
an IME in this state and it will generally look disabled
but still be available in the IME list and once selected
switched to the enabled state.

Change-Id: I77f01c70610d82ce9070d4aabbadec8ae2cff2a3