Fix "leak" in FrameBuilder.textureLayer test

DeferredLayerUpdater always did a post to delete itself, which
would result in the test thinking it had leaked an object since
it wasn't deleted when it returned. Fix this by deleting immediately
if we're already on the right thread.

Remove RenderState's requireGlContext assert as it's now
covered by GpuMemoryTracker, which is also more test friendly.
RenderState's assert required an actual EGL context, which we
don't mock away in unit tests.

Change-Id: Ic23eb54e7151355f7acca483d7464350c9d6a87f