Add APK targets to build libjingle tests for Android.

webrtc/system_wrappers/source/field_trial_default.cc

 // Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 //
 // Use of this source code is governed by a BSD-style license
 // that can be found in the LICENSE file in the root of the source
 // tree. An additional intellectual property rights grant can be found
 // in the file PATENTS.  All contributing project authors may
 // be found in the AUTHORS file in the root of the source tree.
 //
 
 #include "webrtc/system_wrappers/include/field_trial.h"
@@ skipping 40 matching lines @@
       return field_value;
   }
   return std::string();
 }
 
 // Optionally initialize field trial from a string.
 void InitFieldTrialsFromString(const char* trials_string) {
   trials_init_string = trials_string;
 }
 
+const char* GetFieldTrialString() {
+  return trials_init_string;
+}
+
 }  // namespace field_trial
 }  // namespace webrtc