Simplify haptic-only ringtone logic

The RingtoneFactory.getHapticOnlyRingtone() method used
config_defaultRingtoneVibrationSound config to parse
the haptic-only ringtone Uri and load Ringtone.

Since that config is not used for any devices. Remove
the getHapticOnlyRingtone method, use null for Uri and
ringtone for haptic-only case.

Flag: EXEMPT refactor
Bug: 354882163
Test: atest RingerTest
Change-Id: Ifdb684f7d0af4bc7fb6819df6414508c09057d26