Update gradle version

[opacam]

To solve the ci errors when trying to download the corresponding build tools which seems to not exist anymore...

Resolves: #1506

[ghost]

My test build with python 2 / API 19 with ndk-bundle 19 / openjdk8 was successful with this new change. (For anyone who wants to reproduce what I ran: sudo p4aspaces cmd p4a-py2-api19-ndkbundle testbuild --p4a https://github.com/opacam/python-for-android/archive/fix-ci-gradle.zip)

[ghost]

I did an additional build of the testapp_keyboard with Python 3 non-crystax / API 28 + NDK 21 / openjdk8 (p4aspaces' p4a-py3-api28ndk21 env) with this pull request because you know, I don't have much better things to do... and that one also worked. Haven't tested the .apk though, but the build runs through fine including the gradle step.

[opacam]

I did test an apk at run time and it seems to work as expected (only python2 tested but it should work for python 3 because I tested with the changes made in pr #1460, so both versions of python shares the same build system). The nature of the failure makes me think that all the ci builds made after the removed build tools will fail, unless restored the build tools...I don't know if we can rely on this...

The only visible difference (in functionality) I find is that the resulting apk's path has been changed into a sub folder (debug or release).

I must say that with this pr the most of the gradle dependencies will be downloaded from google...this will make the build system more reliable? hope that yes...

[inclement]

Thanks!