init: Use ImageManager for DSUs.

The first_stage_mount code for DSUs predates both the DSU name and the
ImageManager abstraction. Move this code to ImageManager, so it can be
shared with overlayfs/scratch handling. And while we're here, rename GSI
to DSU for clarity.

Bug: 134949511
Test: dsu works
Change-Id: I9ef374bccc6cdbe2ada88baef4e7c0bc81b1e85e