fs_mgr: overlayfs: fsck scratch filesystem

Enable fsck phase for scratch filesystem.  Solved a resulting
problem where the end of the fsck executable would result in a
SIGHUP to the caller.

Add some debugging details from adb-remount-test.sh script

Test: adb-remount-test.sh
Bug: 138577868
Bug: 139283818
Bug: 139226412
Change-Id: I129d86b7cf4e938001ba6db8d03247bc9903988d