Add RC test app. (#3819)

* Add RC test app.

* Add private header files to podspec used by 2P.

* Rename FIRRemoteConfig_Internal.h to FIRRemoteConfig_Private.h.

* Fix style.

* Add copyright notice.

* Fix relative path to RCNConfigSettings for tests.

* Fix style.

* Rename to FIRRemoteConfig_Private.h for unit tests.

* Rename to coorect relative path.

FirebaseRemoteConfig.podspec

@@ -30,6 +30,7 @@ app update.
   s.source_files = base_dir + '**/*.[mh]'
   s.requires_arc = base_dir + '*.m'
   s.public_header_files = base_dir + 'Public/*.h'
+  s.private_header_files = base_dir + 'Private/*.h'
   s.pod_target_xcconfig = {
     'GCC_C_LANGUAGE_STANDARD' => 'c99',
     'GCC_PREPROCESSOR_DEFINITIONS' =>