Implement #8323587, #8323342, #8323590: new features.

8323587: Add feature for supporting app widgets
8323342: Add feature for replacing the home screen
8323590: Add feature for supporting input methods

The app widget service looks for the app widget feature
and refuses to work if it doesn't exist.  I didn't do
this for the input method service because some devices
will probably want to still make use of that mechanism
without supporting third party input methods.

Change-Id: Ia358ccc6059d43f163d74810c7cbe9568a67ede9