Setup mobile config and create experiment for using overflowInset in Android

Summary:
The previous diff shows how `overflowInset` could improve hit test algorithm performance. This diff adds experiment to it in order to:

1. Understand the perf improvement in production
2. Provide quick way to rollback in production -- the changes are used by FB4A as well as VR
3. The changes will pass more instructions over JNI, which may have impact on perf.

To share the MC param values in both Java and C++ side, the value is hosted by `ReactFeatureFlags` and fetched in `FbReactInstanceHolder`.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D33179310

fbshipit-source-id: 327100d41f7b5a668ff0d2afabcdd1fc16cb5a18

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -94,6 +94,17 @@ public class ReactFeatureFlags {
     return mapBufferSerializationEnabled;
   }
 
+  /** Feature Flag to use overflowInset values provided by Yoga */
+  private static boolean useOverflowInset = false;
+
+  public static void setUseOverflowInset(boolean enabled) {
+    useOverflowInset = enabled;
+  }
+
+  public static boolean doesUseOverflowInset() {
+    return useOverflowInset;
+  }
+
   /** Enables Fabric for LogBox */
   public static boolean enableFabricInLogBox = false;