Adding an image cache to be behind drawLayers.

This is intended to replace the shadow GLImage array in BufferQueueLayer, since
I think this is simpler, and I don't fully trust BufferLayerConsumer's locking
model so that we could safely hold a unique_ptr<GLImage> there while passing a
pointer to RenderEngine::drawLayers.

Also, putting RenderEngine behind a binderizable interface is a bit of
a stretch goal for profiling, so EGLImage operations will need to be
behind the interface.

Bug: 118461793
Change-Id: Ibde79abd722ecb48ddf1288bc27008e07e1dc0c4
Test: librenderengine_test