SF: Connect the VDS scratch surface producer

- VirtualDisplaySurface has a BufferQueue for producing scratch
  buffers when doing mixed composition for a virtual display.
- Connect/disconnect the producer end of this BufferQueue in the
  VirtualDisplaySurface constructor/destructor to avoid using it in a
  disconnected state.

Bug 24733179

Change-Id: I87dab9ff4f46d422b7796670b0b4d17b7fd09165