Fix canonicalizing /sdcard

We now transform the realpath of /sdcard with FUSE mounts from
/mnt/user/<userid>/emulated/<userid> to
realpath of /storage/emulated/<userid> which is what is expected
without FUSE when matching with VolumeInfo#path.

Test: atest
android.os.storage.cts.StorageManagerTest#testGetExternalUuidForPath
Bug: 135341433

Change-Id: I8ffe8ac7109329d360fe18ce3e6ed227672b77bc