Support getStorageVolume() for synthetic volumes.

The MediaStore.VOLUME_EXTERNAL volume is a synthetic collection of
all currently mounted storage volumes, so if someone wants to find
the underlying StorageVolume for one of these Uris, we need to query
to resolve the real volume name.

Bug: 153664437
Test: atest android.provider.cts.MediaStoreTest
Change-Id: I7b28ba3bfd5b7a4cc577830940de4a2e1cbd1932