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
Merge commit 'refs/changes/26/50826/2' of http://review.cyanogenmod.org/CyanogenMod/Superuser into cm-10.2