Fix gesture preview image visibility not set correctly.

- update for the preview image is done only in onStart()/Stop().
Move the handling to onPause()/onResume() instead so that visibility
updates will also be done when switching between pages.
- remove the obsolete GesturePreference class.

Change-Id: I743a9d41f17aab3303201e11e5bba708ca5810ae
Fix: 63783217
Test: make RunSettingsRoboTests

res/values/attrs.xml

@@ -138,12 +138,6 @@
         <attr name="allowDividerBelow" format="boolean" />
     </declare-styleable>
 
-    <!-- For GesturePreference -->
-    <declare-styleable name="GesturePreference">
-        <attr name="animation" format="reference" />
-        <attr name="preview" format="reference" />
-    </declare-styleable>
-
     <declare-styleable name="VideoPreference">
         <attr name="animation" format="reference" />
         <attr name="preview" format="reference" />