[master_chooser] bugfix launch mode for the master chooser.

The singleTask launch mode now causes startActivityForResult to immediately cancel
and return. Even though the master chooser would continue, any subsequent calls to finish
would not return any meaningful activity result because it had already returned a cancelled
result as it started.

http://stackoverflow.com/questions/8960072/onactivityresult-with-launchmode-singletask

This fixes #231. It may also have something to do with rosjava/android_remocons#61.

android_10/AndroidManifest.xml

@@ -17,7 +17,7 @@
         <activity
             android:name="org.ros.android.MasterChooser"
             android:label="@string/app_name"
-            android:launchMode="singleTask"/>
+            android:launchMode="standard"/>
 
         <service android:name="org.ros.android.NodeMainExecutorService"
                  tools:ignore="ExportedService">