fs_mgr_overlayfs: MapScratchPartitionIfNeeded always tries to create scratch

Simplify the logic of MapScratchPartitionIfNeeded to just map the
scratch partition unconditionally. If there is a "scratch" logical
partition available, we just assume it must be needed later to
facilitate overlayfs.

Going through the fstab & fs_mgr_overlayfs_candidate_list to determine
if overlayfs is needed or not might be inaccurate as the fstab passed into
MapScratchPartitionIfNeeded is incomplete (it includes only the
"first_stage_mount" entries). So the scratch device might not get created
even though there are some "late mount" partitions that need the scratch
device to facilitate overlayfs.

Bug: 306124139
Test: adb-remount-test
Change-Id: I8590750e822219dec3d7d3c8648e9c9a5a32f68c