Fix multi-user and multi-storage with FUSE

Fix multi-user and multi-storage with FUSE

Up until now, the FUSE mount logic has made two assumptions:
1. The primary external volume is an emulated volume on /data/media
2. Only the primary user is running, as user zero

These assumptions are fixed by the following changes
creating an EmulatedVolume per Android user and changing the
VolumeBase id format to append the user to the id, so
s/emulated/emulated-0/. This allows us mount separate volumes per user

Some additional refactorings to re-use/clean up code.

Test: adb shell sm set-virtual-disk and partition disk operations work
even after setting up a work profile
Bug: 135341433

Change-Id: Ifabaa12368e5a591fbcdce4ee71c83ff35fdac6b