Fix the description of one-handed mode doesn't change immediately in

Gestures page after users turn on/off in Quick Settings

Add ContentObserver URI for observing when settings key
"one_handed_mode_enabled" changes.

Bug: 230041450

Test: manually verified on System > Gestures >
One-handed mode item when turned  on/off in Quick Settings

Test: make RunSettingsRoboTests ROBOTEST_FILTER=
"com.android.settings.gestures.
OneHandedEnablePreferenceControllerTest"

Change-Id: Id6bb53692181dfc3c8a1006017b5a22462dc4ad9