Build improvements

[ligi]

mainly wanted the "attach resources" and did the other stuff in the process

[tingyik90]

Hi @ligi, thanks for this pull request! But I am rather reluctant to use gradle 6.0.1 since the latest stable is 5.4.1. I've upgraded gradle in commit 60d2891.

Other changes that you made are for the app section, which is not really a concern since that is for demo purpose only.

[ligi]

thanks for looking into this. Do not yet understand where 5.4.1 is the latest stable - AFAIK it is 6.0.1: https://github.com/gradle/gradle/releases
Anyway - do not care for the gradle version that much - but really want attached sources.

[tingyik90]

I'm looking at https://developer.android.com/studio/releases/gradle-plugin#updating-gradle, where there seems to be a limit up to gradle 5.6.4. So I'd like to keep it as default that is suggested by Android Studio.

As for attached sources, I'm not familiar with what a7e3136 will generate. Do you mind pointing out some explanation of what it does? Sorry about that as I'm not familiar with gradle besides of what I normally use. I'll look into it this week as it's 2 AM local time now. Thanks.

[ligi]

it publishes the sources as artifacts - so when consuming the library I can click on the function and see the sources - quite important to me