Update android build instruction for webrtc unity plugin.

webrtc/examples/unityplugin/ANDROID_INSTRUCTION

 Instruction of Running webrtc_unity_plugin on Android Unity
 
 1. On Linux machine, compile target webrtc_unity_plugin.
    Checkout WebRTC codebase: fetch --no-hooks webrtc_android
    If you already have a checkout for linux, add target_os=”android” into .gclient file.
       Run gclient sync
+   Modify files src/build/android/android_only_jni_exports.lst and src/build/android/android_only_explicit_jni_exports.lst to expose all functions. Namely, change "global" section to "*", and remove "local" section. Otherwise, Unity C# code will not be able to access the functions defined in the plugin. If you do this step after you create a build folder, you may have to clean and recreate the build folder.
       Run gn args out/Android, and again set target_os=”android” in the args.gn
-   Modify file src/build/android/android_only_jni_exports.lst, to expose all functions. Namely, change "global" section to "*", and remove "local" section. Otherwise, Unity C# code will not be able to access the functions defined in the plugin.
-   Run ninja -C out/Android webrtc_unity_plugin
+      Run ninja -C out/Android webrtc_unity_plugin
 
 2. On Linux machine, compile target libwebrtc_unity under webrtc checkout. This is the java code for webrtc to work on Android.
 
 3. Copy libwebrtc_unity.jar and libwebrtc_unity_plugin.so into Unity project folder, under Assets/Plugins/Android folder.
 
 4. Rename libwebrtc_unity_plugin.so to libjingle_peerconnection_so.so. Again, this is hacky, and the purpose is to let the java code in libwebrtc.jar to find their JNI implementation. And simultaneously, in your C# wrapper script for the native plugin libjingle_peerconnection_so.so, the dll_path should be set to “jingle_peerconnection_so”.
 
 5. In the Unity Main Scene’s Start method, write the following code to initialize the Java environment for webrtc (otherwise, webrtc will not be able to access audio device or camera from C++ code):
 
 #if UNITY_ANDROID
         AndroidJavaClass playerClass = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
         AndroidJavaObject activity = playerClass.GetStatic<AndroidJavaObject>("currentActivity");
         AndroidJavaClass webrtcClass = new AndroidJavaClass("org.webrtc.PeerConnectionFactory");
         if (webrtcClass != null)
         {
             webrtcClass.CallStatic("initializeAndroidGlobals", new object[2] { activity, false });
         }
 #endif
 
-6. Compile the unity project into an APK, and decompile the apk using apktool you can download from internet. And copy the AndroidManifest.xml to the Assets/Plugins/Android folder, and add two lines:
-  <uses-permission android:name=”android.permission.RECORD_AUDIO” />
-  <uses-permission android:name=”android.permission.CAMERA” />
+6. Compile the unity project into an APK, and decompile the apk using apktool that you can download from https://ibotpeaches.github.io/Apktool/
+    Run apktool d apkname.apk.
+Then copy the AndroidManifest.xml in the decompiled folder to the Assets/Plugins/Android folder, and add two lines:
+  <uses-permission android:name="android.permission.RECORD_AUDIO" />
+  <uses-permission android:name="android.permission.CAMERA" />
 
   The purpose of using apktool is to get a well-written android manifest xml file. If you know how to write manifest file from scratch, you can skip using apktool.
+
+7. Compile the unity project into an APK again and deploy it to an android device.