fs_mgr: overlayfs: fs_mgr_overlay_setup true if already mounted

fs_mgr_overlayfs_setup return false second time around if all
are setup. It should report true if candidate list is all done.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: Ic9e16489e2c8ac4d3de91fcf8cfceedc23595e94