SR: Fix datamedia detection

Devices with secondary storage but no primary storage should be
detected as datamedia.

Rules for determining primary storage are:

 * Static mounts named /sdcard are primary storage except when the
   type is declared as datamedia.

 * Static mount points beginning with /mnt/media_rw/sdcard are
   primary storage unless followed by a non-zero value.

 * Dynamic mount points beginning with sdcard are primary storage
   unless followed by a non-zero value.

Change-Id: Ica6e967f98e4506d85095a58b6ede774db64956d