SF: Make ExternalTexture mockable

Expose GraphicBuffer properties via ExternalTexture class. 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: I2e7f6bee28314b70eac477cfadbf9f94c4d70339