Treat composition frames with no layers as using GLES composition

When there are no window layers for a display, SurfaceFlinger clears
the undefined region using GLES. Some of the places that check for
GLES composition weren't considering this special case, in particular:

- We were skipping the eglSwapBuffers() on these frames.
- We were putting VirtualDisplaySurface in HWC-only composition mode.

This change centralizes the logic for this special case.

Bug: 10957068
Change-Id: I2deaf2ed101e8ea76708862a6bb67751b6078794