remount: Refactor fs_mgr_overlayfs_scratch_device().

This splits fs_mgr_overlayfs_scratch_device into two new methods. The
first, GetScratchStrategy, returns an enum detailing exactly how the
device will find space for scratch:
 - via system_<other>,
 - via super_<other>, or
 - via a dynamic partition.

The second method, GetScratchDevice, uses the strategy to either return
the underlying block device, or look up a dynamic partition.

fs_mgr_overlayfs_scratch_device will be removed completely in a separate
patch.

Bug: 134949511
Test: adb remount and adb_remount_test.sh
Change-Id: Ic7e3815ada4adaf5fd7c19b84dc22249f3dcc881