Update VTS to use ExternalTexture impl

We want to expose GraphicBuffer properties via
ExternalTexture class and within, SurfaceFlinger access the
buffer via this proxy interface. This allows us to inject
and mock GraphicBuffers as needed. Specifically this will
be used to recreate layer state from transaction traces.

Test: compiles
Bug: 200284593

Change-Id: If2b6c43bb5b302fbc49dfa49d6ac1f79f92f30f7