Move eglSwapBuffers out of HWComposer

Commit 86303204 moved the eglSwapBuffers fallback (for devices with no
HWC implementation) from DisplayHardware to HWComposer. But HWComposer
only knows about the framebuffer EGL display and surface handles if
there is a HWC, so it was always passing bogus handles.

This change moves the eglSwapBuffers fallback up to SurfaceFlinger,
which has access to the framebuffer EGL handles.

Bug: 6886613
Change-Id: Iad3f5ff7c90ee48d7053999e6a4548d6794b6ebd