DeskClock: Change action intent back to GET_CONTENT

- This this needed as a CMCC requirement to show the CMFM
and Sound Recorder as options in the DocumentsUI drawer
when the user chooses to set a Music file as their alarm
ringtone
- MANAGE_DOCUMENTS permission is requirement to retain access
to the URI returned by DocumentsUI when the user picks a file
from DocumentsUI internal storage
- The platform signature is required for MANAGE_DOCUMENTS

Revert "deskclock: take persistent grants of external uris"

Change-Id: Idd970801682b73b3f9e66d6651dcc80d208c05f2