fs_mgr: get fs_mgr_mount_all to call fs_mgr_overlayfs_mount_all

After fs_mgr_mount_all has added the resources, let
fs_mgr_overlayfs_mount_all to read-only mount overlayfs over the
possible system partitions.

Test: compile
Bug: 109821005
Bug: 110985612
Change-Id: I7101a04c57de1a26283b1523636d07fa8e19ffc7