fs_mgr: overlayfs: if existing scratch mountpoint is system_empty

On first mount check if overlays are setup, if not and there is
no space, then umount and make a filesystem and try again.

Test: adb_remount_test.sh
Bug: 119885423
Change-Id: I3a76ff5b53474976f72e9db326b26838f7df5ba8
1 file changed