fs_mgr: add overlayfs handling for squashfs system filesystems

/cache/overlay directory in support of overlayfs mounts on userdebug
and eng devices.  Overlayfs in turn can be capable of supporting
adb remount for read-only or restricted-storage filesystems like
squashfs or right-sized (zero free space) system partitions
respectively.

Test: compile
Bug: 109821005
Bug: 110985612
Change-Id: I3ece03886db7cc97f864497cf93ec6c6c39bccd1