Remove RenderThread from EglManager

Refactor to make EglManager re-usable.

Test: hwuiunit passes, hwuimacro works

Change-Id: Ie8e9398c703fada1dc5d8baca5f42485eadea202
diff --git a/libs/hwui/EglReadback.cpp b/libs/hwui/EglReadback.cpp
index a836afe..2d5367b 100644
--- a/libs/hwui/EglReadback.cpp
+++ b/libs/hwui/EglReadback.cpp
@@ -58,7 +58,7 @@
 CopyResult EglReadback::copyGraphicBufferInto(GraphicBuffer* graphicBuffer,
                                                  Matrix4& texTransform, const Rect& srcRect,
                                                  SkBitmap* bitmap) {
-    mRenderThread.eglManager().initialize();
+    mRenderThread.requireGlContext();
     // TODO: Can't use Image helper since it forces GL_TEXTURE_2D usage via
     // GL_OES_EGL_image, which doesn't work since we need samplerExternalOES
     // to be able to properly sample from the buffer.