DeskClock: Fix behavior when setting external media file for alarm tone

Repro:
1) Set an alarm
2) Click on sound
3) Select Ringtone then select Music and press OK
4) In DocumentsUI picker, go to overflow menu and select "Show internal storage"
5) Open drawer and select Internal storage
6) Navigate to and select a music file e.g. "/Downloads/test.mp3"

Observe that when the alarm rings, the music file selected isn't played, instead
alarm falls back to fallback ringtone

Change-Id: I6bf19c0e94fc5fe6415de9c291aa938f6043e380