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
diff --git a/private/file_contexts b/private/file_contexts
index 2087a36..6c75385 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -518,6 +518,11 @@
 # LocalTransport (backup) uses this subtree
 /cache/backup(/.*)?		u:object_r:cache_private_backup_file:s0
 
+#############################
+# Overlayfs support directories
+#
+/cache/overlay(/.*)?            u:object_r:overlayfs_file:s0
+
 /data/cache(/.*)?		u:object_r:cache_file:s0
 /data/cache/recovery(/.*)?	u:object_r:cache_recovery_file:s0
 # General backup/restore interchange with apps