fs_mgr: overlayfs create scratch using a Dynamic Logical Partition

The default is a valid /mnt/scratch/overlay/ or /cache/overlay/
directory, with .../<mount_point>/upper and .../<mount_point>/work,
associated with each system partition <mount_point>.  Add support
for Dynamically Resized Partitions to create /mnt/scratch.

Test: manual
Bug: 109821005
Change-Id: I5e3f66778ea9e937c839e25541fedb6fbf34ab50