Set correct quota project ID on application directories.

Use PrepareAppDirsFromRoot() to setup the quota project ID on
application-specific directories correctly. App directories use
AID_EXT_GID_START + their application ID offset, whereas cache
directories use AID_CACHE_GID_START. This is consistent with the GIDs
sdcardfs used to label these directories with.

Bug: 146419093
Test: verified project IDs with lsattr -p
Change-Id: Idca8a30d185012efb0d19ceb9b346b9a4de34f18