[SettingsProvider] clear binder identity when updating the ringtone cache

Otherwise non-system callers can set the ringtone but cannot update the
ringtone cache. This doesn't affect a normal user journey (user setting Ringtone via the Settings app). It only affects CTS tests and third-party apps that tries to update the system ringtone.

BUG: 297847899
Test: atest android.media.audio.cts.RingtoneManagerTest

Change-Id: I15ed4bd3f9d49b0edeea0c752f37ad8b8e421473