Only use access_disabled if SUPERUSER_EMBEDDED.

It is not possible to reenable Superuser when non embedded. su will reject.

Change-Id: If440d9d758c21c61a2d7c0bef132eb716b494c5d
Make emulated storage mounting consistent to Zygote.

- Mount in private namespace
- Mount in forked child
- Mount /storage/emulated/legacy

All of this is optional and must be enabled by setting the environment
variable MOUNT_EMULATED_STORAGE.

Change-Id: Ied9c4485e58ab26ba8e6eafcbe05bb75fe6fdc52