cmfm: Use FileProvider for serving files

 * On N, apps can no longer share file:// uris to other packages.
   Apps that wish to do this must use a FileProvider to
   generate content uris which can then be shared.
 * This patch only currently handles the external storage case and does
   not consider arbitrary filesystem paths or root access. This could
   be implemented in the future, if necessary. The common case of
   opening a media file is a good start, though.

Change-Id: I83aef923dfa26173171a13c0881aeb43337b4e47