fs_mgr: overlay: umount scratch more aggressively

For specific teardown operations, if we mount scratch with the
intention of it being temporary to strike out content for a specific
partition, umount it.  Otherwise it could interfer with fastboot.

Test: adb-remount-test.sh
Bug: 120034852
Change-Id: Ieff3f6ea0bcda5eb540e9acf6cab8a56d1453b3e