Use HWC_DISPLAY_PRIMARY and _EXTERNAL

Some of the code in the SurfaceFlinger and VR composer assume that the
composer has display IDs HWC_DISPLAY_PRIMARY and HWC_DISPLAY_EXTERNAL
for those specific displays. Modify test to follow the requirement.

Bug: 64446190
Test: Run the test on Marlin
Change-Id: I7fef17158d62bd7cd242b97d37724a229f9c7418
diff --git a/services/surfaceflinger/tests/fakehwc/FakeComposerClient.h b/services/surfaceflinger/tests/fakehwc/FakeComposerClient.h
index 294abb2..2a5a8ad 100644
--- a/services/surfaceflinger/tests/fakehwc/FakeComposerClient.h
+++ b/services/surfaceflinger/tests/fakehwc/FakeComposerClient.h
@@ -19,6 +19,9 @@
 #include "ComposerClient.h"
 #include "RenderState.h"
 
+// Needed for display type/ID enums
+#include <hardware/hwcomposer_defs.h>
+
 #include <utils/Condition.h>
 
 #include <chrono>
@@ -40,6 +43,13 @@
 
 namespace sftest {
 
+// NOTE: The ID's need to be exactly these. VR composer and parts of
+// the SurfaceFlinger assume the display IDs to have these values
+// despite the enum being documented as a display type.
+// TODO: Reference to actual documentation
+constexpr Display PRIMARY_DISPLAY = static_cast<Display>(HWC_DISPLAY_PRIMARY);
+constexpr Display EXTERNAL_DISPLAY = static_cast<Display>(HWC_DISPLAY_EXTERNAL);
+
 class FakeComposerClient : public ComposerBase {
 public:
     FakeComposerClient();