Add Global Settings key for HEARING_DEVICE_LOCAL_NOTIFICATION

* Move HEARING_DEVICE_LOCAL_AMBIENT_VOLUME from System to Global.
Since bluetooth device data is persist across users, so hearing device settings should also persist to all users.

Flag: com.android.settingslib.flags.hearing_devices_ambient_volume_control
Flag: com.android.settingslib.flags.hearing_devices_input_routing_control
Bug: 357878944
Bug: 349255906
Test: atest SettingsBackupTest
Change-Id: I09df4b2f5b7d98887c049c7fa2e27770748daeee