briefcase: Android gradle build fails for application

I was trying to build a simple dummy app using toga and packaging it with briefcase for different platforms. The simple app opens a blank window (source code below)

import toga
from toga.style import Pack
from toga.style.pack import COLUMN, ROW
class Freedom(toga.App):
    def startup(self):
        # Create a main window with a name matching the app
        self.main_window = toga.MainWindow(self.name)

        # Create a main content box
        main_box = toga.Box()

        # Add the content on the main window
        self.main_window.content = main_box

        # Show the main window
        self.main_window.show()

def main():
    return Freedom('Freedom', 'com.example.freedom')

The build succeeds for macOS and linux platforms and runs fine on these platforms (i didn’t try others yet) but fails for android. I am working on Mac OS Sierra with python 3.6, toga.version 0.3.0.dev8 , briefcase.version 0.2.1 To setup the development environment for android I installed Android Studio (v 3.0.1) downloaded sdk 15 and 25. I set up the ANDROID_HOME environment and PATH variables as described in https://media.readthedocs.org/pdf/briefcase/latest/briefcase.pdf (page 14, android section)

Running python setup.py android successfully creates the android folder with the output

Installing collected packages: freedom
  Running setup.py install for freedom ... done
Successfully installed freedom-0.0.1
 * No icons defined - using default...
 * No splash screen defined...

Installation complete.

To compile, install and run the project on your phone:

    $ cd android
    $ ./gradlew run

The error occurs when using gradlew. If I run gradlew build or gradlew run, the process ends with the error message

Problem occurred in toga/handlers.py
Node: YieldFrom(value=Call(func=Attribute(value=Name(id='asyncio', ctx=Load(), lineno=19, col_offset=23), attr='sleep', ctx=Load(), lineno=19, col_offset=23), args=[
    Name(id='delay', ctx=Load(), lineno=19, col_offset=37),
  ], keywords=[], lineno=19, col_offset=23), lineno=19, col_offset=12)
:buildPythonDebug FAILED

FAILURE: Build failed with an exception.

* Where:
Build file 'freedom/android/build.gradle' line: 15

* What went wrong:
Execution failed for task ':buildPythonDebug'.
> Process 'command 'voc'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 16.915 secs

Any idea what might be going wrong?