Create and support isolation for /data/storage_area

Creating the directory /data/storage_area (which will store each user's directory
of app directories of storage areas) on startup, and adding this directory
/data/storage_area to the tmpfs mirror so it can be protected by app data isolation
in zygote.

Bug: 325121608
Test: atest StorageAreaTest
Change-Id: Ia938e89fd8b794dbcbb844b01b790db7c0a62319