fs_mgr: add /mnt/scratch to possible overlayfs support directories

A default set of options are available, but can override in a fstab
overlay entry with upperdir=, lowerdir= to the same mount point,
workdir=.  The default is a valid /mnt/scratch/overlay/
or /cache/overlay/ directory, with .../<mount_point>/upper and
.../<mount_point>/work, associated with each system partition
<mount_point>.

Test: manual
Bug: 109821005
Change-Id: I5662c01fad17d105665be065f6dcd7c3fdc40d95
diff --git a/private/file_contexts_overlayfs b/private/file_contexts_overlayfs
index 00902c2..e472fad 100644
--- a/private/file_contexts_overlayfs
+++ b/private/file_contexts_overlayfs
@@ -4,3 +4,6 @@
 /cache/overlay/(system|product)/upper       u:object_r:system_file:s0
 /cache/overlay/(vendor|odm)/upper           u:object_r:vendor_file:s0
 /cache/overlay/oem/upper                    u:object_r:vendor_file:s0
+/mnt/scratch/overlay/(system|product)/upper u:object_r:system_file:s0
+/mnt/scratch/overlay/(vendor|odm)/upper     u:object_r:vendor_file:s0
+/mnt/scratch/overlay/oem/upper              u:object_r:vendor_file:s0