SurfaceFlinger: Don't handle effects if a buffer is available

Currently we do not support drawing effects and  buffers in the
same layer. Provide priority to drawing the buffer and then try
and handle both as a follow up.

Test: can add widgets to launcher
Test: go/wm-smoke
Bug: 245052266
Change-Id: I4399e954b6116fd8e5ee618c20fb3c604b657e04