Vold will always bind mount obb and data dirs to lowerfs

Vold will always bind mount obb and data dirs to lowerfs

So shell / root will always access to them directly not via fuse.
And zygote will be unmount these directories to prevent them being
abused for leaking app visibility.

Also, /mnt/androidwritable is not very useful now as it's the same as
/mnt/installer, but we should make shell / root to access /mnt/androidwritable
later and /mnt/installer should only access obb but not data dir.

Bug: 182997439
Test: Able to boot without errors
Test: df on /sdcard/Android/data shows it's no on fuse.
Change-Id: I2ad10b1e80c135f637d37ddf502ee010f89f4946