SF/CE: Clean up header dependencies and combine mocks

CompositionEngine has for a while had a weird dependency on SF
DisplayHardware sources, even though SF depends on CompositionEngine.

This made it impossible to share mocks, so a few were duplicated.

This MINIMALLY cleans things up, by creating filegroups for
libsurfaceflinger_backend_sources, and
libsurfaceflinger_backend_mock_sources, as well as cc_library_header
targets for both.

A more complete cleanup is left to the future, which would require
reorganizing the files.

Bug: None
Test: m surfaceflinger_service_fuzzer
Test: atest libcompositionengine_test libsurfaceflinger_unittest
Flag: EXEMPT refactor

Change-Id: I9d32d715b1a693093e59c8150203c0d941aa0fb2
diff --git a/services/surfaceflinger/CompositionEngine/tests/DisplayTest.cpp b/services/surfaceflinger/CompositionEngine/tests/DisplayTest.cpp
index 9c0e62c..416001e 100644
--- a/services/surfaceflinger/CompositionEngine/tests/DisplayTest.cpp
+++ b/services/surfaceflinger/CompositionEngine/tests/DisplayTest.cpp
@@ -36,10 +36,10 @@
 #include <ui/Rect.h>
 #include <ui/StaticDisplayInfo.h>
 
-#include "MockHWC2.h"
-#include "MockHWComposer.h"
-#include "MockPowerAdvisor.h"
 #include "ftl/future.h"
+#include "mock/DisplayHardware/MockHWC2.h"
+#include "mock/DisplayHardware/MockHWComposer.h"
+#include "mock/DisplayHardware/MockPowerAdvisor.h"
 
 #include <aidl/android/hardware/graphics/composer3/Composition.h>