Move vsoc hwcomposers into subdirectories

This is a restructuring to make room for the incoming virtio
hwcomposer.  The active hwcomposers will be vsoc and virtio.
The vsoc-future hwcomposer is a hwcomposer that composes frames
on the host-side instead of the guest and is still in active
development, which is why it is renamed to "future". The "legacy"
vsoc hwcomposer is the one in active use.

Change-Id: I9681963504abb1aaae4575f0e3241301fb0c7c4c