Create /data_mirror in init.rc

Mount a tmpfs on top of it, and serve as a mirror of /data/data
and /data/user_de, which will be used when zygote forks and
overlay its CE and DE storage to prevent app can see other applications.

Also, changed /data/user/0 from symlink to bind mount, so
/data_mirror/data_ce/null can just bind to /data/user directly.

Bug: 143937733
Test: Directories are created and mounted
Change-Id: Ic72c47c9d3dc0ffc98510f5bb351eccf76524232