[RenderEngine] Move GLES20RenderEngine creation code to GLES20RenderEngine.

Previously, impl::RenderEngine serves as an abstract of GLES specific render
engine instance, which manages the creation and destruction. However, as public
interface, it should be implementationa agnostic. This patch moves EGL
environment set up code to GLES20RenderEngine.

BUG: 112585051
Test: Build, flash, boot and run part of display verification.
Change-Id: Ie87d96c30999706eaa14c28e0e456300848176d5