Move stylus hover icon setting to core settings

The secure setting STYLUS_POINTER_ICON_ENABLED needs to be read from
ViewRootImpl, which means it is read in each activity when it starts.

Instead of reading the setting from the app, move it to core settings by
adding it to the CoreSettingsObserver so that the setting is
pre-populated by system_server when the activity starts, eliminating a
settings read in each app launch.

Bug: 324429095
Test: manual with stylus
Change-Id: Ib71a204685a8eb99fe356ea749f7a21614f4a59d