Export to Android using Gradle

[amanj120]

Choosing "custom build" will allow users to create a full gradle project for their game including their assets.

[akien-mga]

What does this aim to do that isn't already implemented by the Custom Build feature for Android in Godot 3.2?

For feature proposals, a detailed proposal should be opened on https://github.com/godotengine/godot-proposals to explain the use case and the proposed implementation, so that it can be discussed with the community.

CC @m4gr3d

[amanj120]

Open Issue

[amanj120]

I am going to be closing this PR and instead submit multiple smaller, incremental PR's that will build up to do the same thing. This is to make the code reviews easier, and because having smaller, more focused PR's are generally better practice.