Support a mount mode to bypass FUSE on /storage

When Zygote forks a process with the MOUNT_MODE_PASS_THROUGH
flag, bind mount the pass_through path into /storage for the
process, allowing the process to bypass FUSE.

Bug: 140064376
Test: cat /proc/<pid of app with pass_through flag>/mountinfo shows
the lower fs bind mounted directly on /storage

Change-Id: Ia212c2c71ecfdc1629739ca8af171f332ccfca72