[Multi-user] Remove dir management for non-system users

Per-user CE directories are managed by vold so we don't need to
initialize or apply restorecon on them in our service.

Once the system user is migrated to the its CE directory, we can remove
the remaining restorecon and mkdirs logic.

Accompanies aosp/875352 and aosp/873133

Bug: 121197420
Test: 1) Boot device; check directories exist and labelled properly
for system user; perform backup successfully
2) Add secondary user; check directories exist and labelled properly;
perform backup succesfully

Change-Id: If848da4c9254db87b393fce6cfc4169785bfac51