Offer to "fixup" GIDs used for app data.

Offer to "fixup" GIDs used for app data.

We recently started tracking cached app data using a per-app GID for
the "cache" and "code_cache" directories and their contents.  For
upgraded devices, we ideally want to "fixup" the GIDs of any existing
data while the device is still showing the boot animation, instead of
blocking the user when they unlock the device.

Since all the information we need is available in metadata, we can
update GIDs before the user has unlocked data.  We're pretty paranoid
and we only pivot between the normal app GID and the cache GID; any
other GID values are ignored.

This "fixup" method can also be used in the future to ensure
consistency of the files on disk.  Also fix bug by always using
"fts_path" instead of "fts_accpath" which is based on racy chdir().

Test: /data/nativetest/installd_service_test/installd_service_test
Bug: 34201111, 35084485
Change-Id: Ia52694f3763cba09926082c08f0766477e03e39c