Handle UserShortcutType.GESTURE in AccessibilityManagerService

- Update the in-memory gesture targets in AMS when the Setting changes,
and notify interested clients (flag exempt - no UI can modify the gesture setting while unflagged)
- Handle enabling gesture shortcut type in AccessibilityManager (flag protected)
- Map the GESTURE type to the corresponding Setting and logging enum (flag exempt - see above)
- Have the shortcut chooser dialog read from the gesture Setting when in gesture navigation mode (flag exempt). This has been manually tested to work normally with the flag disabled.

NO_IFTTT= false positive, both regions modified
Note: The flag is in development stage, and it won't be turned on by building.
There is currently no interface that can enable a GESTURE type shortcut.

Test: atest com.android.server.accessibility com.android.internal.accessibility
Bug: 330775538
Flag: android.provider.a11yStandaloneGestureEnabled

Change-Id: I57bfd05c65b09317e6c988315d4481274de0d468