Allow one-handed mode shortcut expand notification

1) Add OneHandedEventCallback for WMShell to register
   callback, and if one handed mode settings set show
   notifications, when user tap one handed mode shortcut
   notify WMShell send KEYCODE_SYSTEM_NAVIGATION_DOWN
   through OneHandedEventCallback with SysUIMainExecutor.

2) Add isReady() flag in OneHandedDisplayAreaOrganizer
   and Simplify the flow of onActivatedActionChanged()
   to prevent potential race problem while auto-enable
   One-handed and immeditately startOneHanded(), however,
   DisplayAreaOrganizer#onDisplayAreaAppeared() may not
   ready for transtion yet, and result mState keep in
   unfinsihed STATE_ENTERING.

3) Adjust the transiton duration from 800ms to 600ms
   800ms is a little bit long to affect timing of
   apply runtime overlay package.

4) DelayExecute setEnabledGesturalOverlay() until
   transition finised and at most delay once.
   - onSwipeToNotificationEnabledChanged() : DelayExecute
   - onEnabledSettingChanged() : DelayExecute
   - Ctor setupGesturalOverlay() : No DelayExecute
   - mState.isTransitioning() : DelayExecute

Test: WMShellUnitTests
Bug: 182425480
Change-Id: I2615c3b30ad95a858510b4bcc73d8f343843fc96