Block SHELL_UID from overlay fabricate and clear shell overlays on boot

Prevents non-root shell from fabricating overlays and removes all
overlays with their creating package as shell on boot.

Prevents unapproved usages of the fabrication API and cleans up
any potentially broken overlays respectively.

Bug: 202768292

Test: sts-tradefed run sts-engbuild-no-spl-lock \
        -m StsHostTestCases -t android.security.sts.Bug_202768292

Change-Id: I184cc498e49c416e184cc0855a8810b57dd08175