ANDROID: virtio-gpu api: context init feature

This feature allows for each virtio-gpu 3D context to be created
with a "context_init" variable.  This variable can specify:

 - the type of protocol used by the context via the capset id.
   This is useful for differentiating virgl, gfxstream, and venus
   protocols by host userspace.

 - other things in the future, such as the version of the context.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>

BUG: 173630595
TEST: boot VM

Change-Id: I6c1c17007b11bb3d71fdcba7078897d2a554e413