Use Configuration#unset() instead of #setToDefaults() for clearing

Now Configuration.EMPTY has fontScale set to 0 instead of default 1.
To get config equal to empty unset() method should be used.
Because of incorrect usage of setToDefaults() there was a black flash
when exiting split-screen because override config was no completely
empty. Fixing that also allows us to simplify things a bit when checking
if override config changed.

Bug: 31533188
Test: manual - long-press overview to enter split-screen, repeat to exit.
Change-Id: I6bf7994ff88ebb42db2e2357b762857710432a58
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index bfa02bb..8c1ffa2 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -734,7 +734,8 @@
         } else {
             mFrozenMergedConfig.offer(new Configuration(mTask.mPreparedFrozenMergedConfig));
         }
-        mTask.mPreparedFrozenMergedConfig.setToDefaults();
+        // Calling unset() to make it equal to Configuration.EMPTY.
+        mTask.mPreparedFrozenMergedConfig.unset();
     }
 
     /**