RMX1921: create and symlink a dummy /metadata/apex

Google moved apex sessions directory from /data/apex/sessions to
/metadata/apex/sessions after commit:

"Move apex sessions directory to /metadata"
https://android.googlesource.com/platform/system/apex/+/36cf4bbac6108898a531aacc102dcc489d791b37



Devices with a mounted metadata partition will have the needed
directories set up by system/core/rootdir/init.rc. Xiaomi devices
on sdm845 do not have a metadata partition out of the box, so things
like "Google Play system update" will fail to install the update.

Therefore, create a dummy directory under /data/vendor/metadata_apex
and symlink it to /metadata/apex.

The reason why the old /data/apex/sessions directory is not used
for the symlink is that apexd will call migrateSessionsDirIfNeeded()
to recursively copy things from the old directory to the new one.
Creating the symlink from /data/apex/sessions may result in
unintended behaviors.

Signed-off-by: Chenyang Zhong <zhongcy95@gmail.com>
Signed-off-by: kaderbava <ksbava7325@gmail.com>