Allow adding activities to the manifest

[bauerj]

This allows users to add their own activities to the manifest file. This can be used to start Java activities using pyjnius, e.g.:

        PythonActivity = autoclass('org.kivy.android.PythonActivity')
        SimpleScannerActivity = autoclass("org.electrum.qr.SimpleScannerActivity")
        Intent = autoclass('android.content.Intent')
        intent = Intent(PythonActivity.mActivity, SimpleScannerActivity)
        activity.bind(on_activity_result=on_qr_result)
        PythonActivity.mActivity.startActivityForResult(intent, 0)

This will be used in Electrum to scan QR codes using an embedded copy of zxing for Android.

[ecdsa]

this works for me

[inclement]

Looks good, thanks.