fastboot: call fs_mgr_overlayfs_teardown() in UpdateSuper

When the update-super command is issued, we want overlayfs overrides
to disappear without a doubt, which includes non-A/B utilizing
/cache/overlay/ tree.  Call fs_mgr_overlayfs_teardown() on successful
return.

Test: adb-remount-test.sh
Bug: 120034852
Change-Id: Ia5cdb797f7e8350b5591a51fc8ae5f323901aee4