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
diff --git a/services/surfaceflinger/DisplayHardware/HWC2.cpp b/services/surfaceflinger/DisplayHardware/HWC2.cpp
index 0ea3340..fb82033 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWC2.cpp
@@ -349,7 +349,7 @@
 
 Error Display::getRequests(HWC2::DisplayRequest* outDisplayRequests,
                            std::unordered_map<HWC2::Layer*, LayerRequest>* outLayerRequests) {
-    uint32_t intDisplayRequests;
+    uint32_t intDisplayRequests = 0;
     std::vector<Hwc2::Layer> layerIds;
     std::vector<uint32_t> layerRequests;
     auto intError = mComposer.getDisplayRequests(