Move image creation out of latchBuffer().

GLImage creation isn't necessary until an external texture needs to be
bound.

This also removes the BufferLayerConsumer::Image class, as it's not
really necessary now that image creation is deferred until the external
texture needs to be bound.

Bug: 116277151
Change-Id: Id9420941e9cf7df11233b427c7a98a947f9738d8
Test: SurfaceFlinger_test, libsurfaceflinger_unittest, go/wm-smoke