CE: Must dequeue a buffer if flipClientTarget requested

To match the logic that always queues a buffer if the HWC has a
flipClientTarget request for the current frame, we must also always
dequeue a buffer, even if no client composition is being performed.

This is an port of an (as yet uncommitted) AOSP change with a ChangeId of
I933cbae2f09f81eef6555b1bb1e5991d2c450930, due to the underlying code
having been refactored.

This version also adds and updates unit tests which were added as part
of the refactoring.

Bug: 151698217
Test: libcompositionengine_test
Test: libsurfaceflinger_test
Change-Id: Ia8a1470affb2596b27986cc4153417f48cf4ed1c