Move ringtone redirection to MediaPlayer.

Way back in API 1 we defined Settings.System.DEFAULT_NOTIFICATION_URI
which redirects through SettingsProvider before finally ariving at
the real underlying ContentProvider, usually MediaStore.

With new SELinux rules, we're no longer allowing the system_server
to hold open FDs to shared storage devices, which causes these
proxied openFile() calls to fail.

To work around this, teach MediaPlayer to resolve the final ringtone
Uri without going through the system.

Bug: 18226181
Change-Id: I40c68617c952c0bb3e939e5084f5b68a35e31ae3