MediaProvider: read real mount point for removable storage

*Note: must give media_rw group access to /mnt/media_rw (770)

Change-Id: Ia6e80c5a13a3da5d9ca43aca055c494bc1957c68
Revert "MediaProvider: read fat volume id from prop"

This reverts commit 49bddf1be910340fc4d10e4c2e57e8a4c9b61815.

Change-Id: I0e622e964cec84e9e734a4659f4d23ef378bf65b