Fix One-handed mode Settings Main Switch does not update

its state correctly

Add Settings.Secure.ONE_HANDED_MODE_ENABLED key observer when
its state changes.

Bug: 191267388

Test: Manual verified on Settings > System > Gestures
> Use One-handed mode.
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
      "com.android.settings.gestures
      .OneHandedMainSwitchPreferenceControllerTest"

Change-Id: I264fda3a8ab0a76e6234199346b6720895b95cd6