Add legacy layout support to FUSE, enforce write.

The legacy internal layout places users at the top-level of the
filesystem, so handle with new PERM_LEGACY_PRE_ROOT when requested.

Mirror single OBB directory between all users without requiring fancy
bind mounts by letting a nodes graft in another part of the
underlying tree.

Move to everything having "sdcard_r" GID by default, and verify that
calling apps hold "sdcard_rw" when performing mutations. Determines
app group membership from new packages.list column.

Flag to optionally enable sdcard_pics/sdcard_av permissions
splitting. Flag to supply a default GID for all files. Ignore
attempts to access security sensitive files. Fix run-as to check for
new "package_info" GID.

Change-Id: Id5f3680779109141c65fb8fa1daf56597f49ea0d