Android setup simplification

[vaind]

Fixes #73

[greenrobot-team]

Reviewing...

[greenrobot-team]

To speak in Gradle terms: instead of the user manually adding the dependency, automatically depend on a project/module ("Flutter plugin") that brings in the dependency. Straightforward solution, I guess.

Anyhow: I failed to generate objectbox.g.dart for the example using flutter pub run build_runner build (succeeds, but no changes; Flutter (Channel beta, v1.11.0, on Microsoft Windows [Version 10.0.18363.476], locale de-DE)). But I suppose Ivan tested that the app still works (I see no reason looking at the dependency graph, why it shouldn't).

[liquidiert]

@greenrobot-team dunno if it helps but I'm using object box 0.4.0 in one of my current projects and I can build my g parts 👌

[greenrobot-team]

@liquidiert Thanks for that! Turns out it failed due to a regression introduced with an unrelated change in the development version.