[testapp] Make a flask testapp...

So we can get an apk via gh-actions

Notes:
  - this way the reviewers can install the flask apk without making the build
  - these changes will be reverted in the next
    commit...since we want a kivy app as our default)

testapps/on_device_unit_tests/setup_test_app.py

@@ -39,7 +39,7 @@
     'apk':
         {
             'requirements':
-                'sqlite3,libffi,openssl,pyjnius,kivy,python3,requests',
+                'sqlite3,libffi,openssl,pyjnius,flask,genericndkbuild,python3,requests',
             'android-api': 27,
             'ndk-api': 21,
             'dist-name': 'bdist_test_app_unittests',