Settings: Fix stylus gestures settings and related NPE

This commit fixes two things:
 - The stylus settings category was not removed on non-stylus devices
 - Include all stylus gestures related preferences in the PreferenceCategory
 - A missing null pointer check caused a NPE when IME selector was triggered
   Issue 6503: "Set up input methods" crashes the Settings application

Patch set 1-2: Initial commit
Patch set 3  : Change variable name to better match category name

Change-Id: I6c0a746b33c4a86b977f443233ed79d626859495