Adjust MTP to reference by specific volume name.

The MediaStore.VOLUME_EXTERNAL volume is a merged view of all storage
devices, and clients working on a specific volume need to focus on
the volume they're interested in.

Bug: 129840030
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I91cee6a96d7f9360e6a93a9a3c389b097b6b9967