fs_mgr: add overlayfs handling for squashfs system filesystems

Logistics for adb remount on developer builds for readonly system
partitions using overlayfs to deal with not being able to remount
with read-write.

Conditions are that filesystem is squashfs, read-only, debug build,
kernel supports overlayfs.  The default is a valid /cache/overlay/
directory, with .../<mount_point>/upper and .../<mount_point>/work,
associated with each system partition <mount_point>.

Will take the sepolicy of context of lowerdir as rootcontext= option.

Add fs_mgr_overlayfs_mount_all() to recurse through
fs_mgr_read_fstab_default(), and checking against /proc/mounts to
add any acceptable overlayfs mounts.  The checking with /proc/mounts
makes sure we do not overlayfs overtop an overlayfs.

Add fs_mgr_overlayfs_setup(const char*, const char* bool*) and
fs_mgr_overlayfs_teardown(const char*, bool*) to respectively setup
or teardown the overlayfs support directories.  Return value
includes success and optional supplied change booleans.

Test: compile
Bug: 109821005
Bug: 110985612
Change-Id: Ie9b4e9ca0f2bdd9275ac2e2e905431636948e131