Access removable volumes through /mnt/media_rw

Due to permissions changes, we now need to access
the underlying filesystem of removable devices in
order to get write access.

Add internalPath to StorageVolume, and have VolumeInfo
set the field on creation.

Bug: 77849654
Test: Can write to emulated sdcard through MTP
Change-Id: I63302ecf2dd2600a1c9f3f6ab106c3695654cbaa