Reduce visibility of functions in ReactHostImpl (facebook#39640)

Summary:
Pull Request resolved: facebook#39640

changelog: [internal] internal

Reducing visibility from public to package only for `ReactHostImpl.getDefaultHardwareBackBtnHandler()` since it's only used within package

Reviewed By: mdvacca

Differential Revision: D49612859

fbshipit-source-id: 3c40888da732f33dc046d9363b08119e707f4ea4

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/runtime/ReactHostImpl.java

@@ -597,7 +597,8 @@ <T extends NativeModule> T getNativeModule(Class<T> nativeModuleInterface) {
     return null;
   }
 
-  public DefaultHardwareBackBtnHandler getDefaultBackButtonHandler() {
+  /* package */
+  DefaultHardwareBackBtnHandler getDefaultBackButtonHandler() {
     return () -> {
       UiThreadUtil.assertOnUiThread();
       if (mDefaultHardwareBackBtnHandler != null) {