RingtoneManager: verify default ringtone is audio

When a ringtone picker tries to set a ringtone through
RingtoneManager.setActualDefaultRingtoneUri (also
called by com.android.settings.DefaultRingtonePreference),
verify the mimeType can be obtained (not found when caller
doesn't have access to it) and it is an audio resource.

Bug: 205837340
Test: atest android.media.audio.cts.RingtoneManagerTest
Change-Id: I3f2c487ded405c0c1a83ef0a2fe99cff7cc9328e