Create directory for snapshots of DE_sys apex data.

This creates the /data/misc/apexrollback directory which will
hold snapshots of DE_sys apex data directories (i.e. it will
hold backups of data from /data/misc/apexdata for particular
apexes).

See go/apex-data-directories for details.

Bug: 141148175
Test: Built and flashed, checked directory was created.
Change-Id: If45377a9b29cc1f52dd08ea1339829e3bb3032e9