inputmethod: Fixup remove stylus preference for non stylus devices

This commit included following changes

 * Move show stylus icon checkbox to Pointer Settings Category
 * Fixup issues stylus preference doesn't remove if not supported
 * Improve the code remove stylus preference if not supported
 * Remove an unnecessary variable introduced on http://review.cyanogenmod.org/#/c/33774/

Change-Id: I0f2e1b5cda00033915bb25902ce970f9eda52048