SF: Test coverage for onInitializeDisplays

Add unit tests to cover SurfaceFlinger::onInitializeDisplays()

Test: "atest libsurfaceflinger_unittest" Passes on Pixel XL
Bug: 74827900
Change-Id: Ife3ef43f7c1eb2593c2e784c030f7ad5db34131e
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index 004bccc..f689537 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -87,6 +87,15 @@
 
     auto setDisplayStateLocked(const DisplayState& s) { return mFlinger->setDisplayStateLocked(s); }
 
+    auto onInitializeDisplays() { return mFlinger->onInitializeDisplays(); }
+
+    /* ------------------------------------------------------------------------
+     * Read-only access to private data to assert post-conditions.
+     */
+
+    const auto& getAnimFrameTracker() const { return mFlinger->mAnimFrameTracker; }
+    const auto& getCompositorTiming() const { return mFlinger->getBE().mCompositorTiming; }
+
     /* ------------------------------------------------------------------------
      * Read-write access to private data to set up preconditions and assert
      * post-conditions.