framework/base: get proper obb mountpoint based on user handle

Due to invalid obb mount point, CTS test case was failing to mount
obb file at /mnt/shell/emulated/0. Added new API to support user
based mount point for obb.

Change-Id: I9f113bd98ab3a4af00dd7f788b2dc3bc45627d6c
CRs-Fixed: 587497