MountService: Fix build && applied few changes

-Allow multiple emulated storages at the same time.
https://github.com/temasek/android_frameworks_base/commit/18de71f914fe20122078a09c98a19e737588d7c8

-Allow mounting of multiple volumes via mass storage (framework part)
https://github.com/temasek/android_frameworks_base/commit/7f8061a743214b268a25d6af4b412d389cb5b774