add: basic webrtc is working

ui/android/app/build.gradle

@@ -45,7 +45,7 @@ android {
         applicationId "com.example.ui"
         // You can update the following values to match your application needs.
         // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
-        minSdkVersion flutter.minSdkVersion
+        minSdkVersion 21
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName

ui/android/app/src/main/AndroidManifest.xml

@@ -30,4 +30,6 @@
             android:name="flutterEmbedding"
             android:value="2" />
     </application>
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
 </manifest>