Add tests for LayerStack::ApproximateMatch

Also:
* Fix a bug in approximate match calculation where the number of
differing fields was always zero.
* Add some documentation.

Bug: 180669092
Test: libcompositionengine_test

Change-Id: I89d6d3f3298615e6b964b053d12127143e2dc060
diff --git a/services/surfaceflinger/CompositionEngine/Android.bp b/services/surfaceflinger/CompositionEngine/Android.bp
index d297d74..08147ed 100644
--- a/services/surfaceflinger/CompositionEngine/Android.bp
+++ b/services/surfaceflinger/CompositionEngine/Android.bp
@@ -106,6 +106,7 @@
         "tests/planner/CachedSetTest.cpp",
         "tests/planner/FlattenerTest.cpp",
         "tests/planner/LayerStateTest.cpp",
+        "tests/planner/PredictorTest.cpp",
         "tests/CompositionEngineTest.cpp",
         "tests/DisplayColorProfileTest.cpp",
         "tests/DisplayTest.cpp",