Handle multi-user mountObb() requests.

Since emulated external storage paths differ based on execution
context, carefully fix up paths for various use-cases:

1. When sending paths to DefaultContainerService, always scope
   OBB paths as belonging to USER_OWNER.
2. When sending paths to vold, always build emulated storage paths
   visible to root.
3. Always use the original untouched path when talking with apps.

Mount OBB containers using shared app GID, so that an app can read
the mount point across users.

Handle legacy paths like "/sdcard" by resolving the canonical path
before sending to MountService.  Move tests to servicestests, and
add tests for new path generation logic.

Bug: 7212801
Change-Id: I078c52879cd08d9c8a52cc8c83ac7ced1e8035e7