Mark CachedSet::mFingerprint as const

Bug: 163076219
Test: make

This field is never updated, and we wouldn't want it to be, either.
Remove its redundant initializer, too.

Change-Id: I18b6d921e52559fdab9def8cd1d22d166cc3c64b
diff --git a/services/surfaceflinger/CompositionEngine/include/compositionengine/impl/planner/CachedSet.h b/services/surfaceflinger/CompositionEngine/include/compositionengine/impl/planner/CachedSet.h
index b0e42b7..c9395ca 100644
--- a/services/surfaceflinger/CompositionEngine/include/compositionengine/impl/planner/CachedSet.h
+++ b/services/surfaceflinger/CompositionEngine/include/compositionengine/impl/planner/CachedSet.h
@@ -100,7 +100,7 @@
 private:
     CachedSet() = default;
 
-    NonBufferHash mFingerprint = 0;
+    const NonBufferHash mFingerprint;
     std::chrono::steady_clock::time_point mLastUpdate = std::chrono::steady_clock::now();
     std::vector<Layer> mLayers;
     Rect mBounds = Rect::EMPTY_RECT;