vrwm: Add support for multiple independent displays

This doesn't change much, just splits ShellView and shuffles code
around.

Display-related things go into DisplayView - everything related to
processing and drawing buffers, hit detection, visibility.

ShellView retains overall state management, touchpad, controller and
is the initial recipient of all incoming frames

The composer library is modified to accept and pass multiple displays

Bug: 35996499
Test: Works for the existing single-display output
Change-Id: Ied5061b4dad9e7d68bb187bf86c3d0f2f3b7a55e