SF: Clean up DisplayDevice::initiateModeChange

Trace the pending mode after successfully setting it on HWC.

Remove unused status_t return value, and redundant warning log. Disallow
nullptr for out parameter.

Bug: 255635711
Test: presubmit
Change-Id: I381e5528d6336c300a80abcebccdd76369126c04
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index c70c1df..a061fca 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -223,9 +223,8 @@
 
     void setActiveMode(DisplayModeId, Fps vsyncRate, Fps renderFps);
 
-    status_t initiateModeChange(const ActiveModeInfo&,
-                                const hal::VsyncPeriodChangeConstraints& constraints,
-                                hal::VsyncPeriodChangeTimeline* outTimeline)
+    bool initiateModeChange(const ActiveModeInfo&, const hal::VsyncPeriodChangeConstraints&,
+                            hal::VsyncPeriodChangeTimeline& outTimeline)
             REQUIRES(kMainThreadContext);
 
     void finalizeModeChange(DisplayModeId, Fps vsyncRate, Fps renderFps)