Don't run flask in threaded mode in webview test app

When flask is run in threaded mode, it creates a new thread to handle
each response. In that case, Java classes found with jnius will use the
system class loader instead of the app class loader since the new thread
has no stack frame back to the app. Normally you'd resolve the classes
first, but the test app is explicitly trying to delay loading the
classes until needed. That won't work from new native threads.

testapps/on_device_unit_tests/test_app/main.py

@@ -85,10 +85,15 @@
     app_flask.TESTS_TO_PERFORM = tests_to_perform
 
     print('Current directory is ', realpath(curdir))
-    if realpath(curdir).startswith('/data'):
-        app_flask.app.run(debug=False)
-    else:
-        app_flask.app.run(debug=True)
+    flask_debug = not realpath(curdir).startswith('/data')
+
+    # Flask is run non-threaded since it tries to resolve app classes
+    # through pyjnius from request handlers. That doesn't work since the
+    # JNI ends up using the Java system class loader in new native
+    # threads.
+    #
+    # https://github.com/kivy/python-for-android/issues/2533
+    app_flask.app.run(threaded=False, debug=flask_debug)
 else:
     # we don't have kivy or flask in our
     # requirements, so we run unittests in terminal