surfaceflinger: remove unused EGL data members

Remove SurfaceFlinger::mEGLDisplay and SurfaceFlinger::mEGLContext.
With this change, SurfaceFlinger does not use EGL directly anymore.
This also allows us to get rid of RenderEngine::getEGLContext.

Test: SurfaceFlinger_test
Change-Id: I7799d935367650508b741e62a53ce325fdc94234