Add an empty libjingle_peerconnection_metrics_default_jni target.

This allows downstream dependencies can add it as a dependency.

BUG=webrtc:6499

Review-Url: https://codereview.webrtc.org/2521183002
Cr-Commit-Position: refs/heads/master@{#15178}

webrtc/api/BUILD.gn

@@ -230,6 +230,9 @@ if (is_android && !build_with_chromium) {
     }
   }
 
+  rtc_static_library("libjingle_peerconnection_metrics_default_jni") {
+  }
+
   rtc_shared_library("libjingle_peerconnection_so") {
     sources = [
       "android/jni/jni_onload.cc",