[SettingsProvider] verify ringtone URI before setting

Similar to ag/24422287, but the same URI verification should be done in
SettingsProvider as well, which can be called by apps via
Settings.System API or ContentProvider APIs without using
RingtoneManager.

BUG: 227201030
Test: manual with a test app. Will add a CTS test.
Change-Id: Ic0ffa1db14b5660d02880b632a7f2ad9e6e5d84b