Add 'Export App Bundle' to Android Export Options

[amanj120]

The final step in moving towards a gradle-based build system that can export Android App Bundles is to add the user interface element. For the 3.x release, and following the contribution guidelines, I think the simplest way to do this is to add a drop down menu to be able to Export as AAB. This option will not work unless Use Custom Build is also selected. For more details on the final scope of this project, read this comment.

The default value for this drop down menu is Export APK. If Use Custom Build is not selected, then Export APK follows the legacy build system. If Use Custom Build is selected and Export APK is selected, then it follows the normal Use Custom Build system.

The PR for the 4.0 branch might look different depending on how much of the legacy build system we want to retain.

We are working on this PR with @m4gr3d, as explained here

This PR builds on top of many previous PRs I've submitted. There are a lot of changes that need to be made to move towards a Gradle-based build system, so I have broken these changes into smaller, more understandable steps.

[m4gr3d]

@akien-mga For the UI, another option is to replace Use Custom Build and Export App Bundle (added by this PR) options with a drop-down that would list the various options for exporting the app.
For example, the options could be:

• Export apk using legacy build
• Export apk using gradle build
• Export aab

The issue with this approach of course if that it would be slightly breaking compatibility as opposed to the current which, while not the best UX, does the job and doesn't break compatibility.
If we want to avoid breaking compatibility for the 3.2.x branch, we can also use the drop-down idea for the PR against master.

[m4gr3d]

Looks great!

[m4gr3d]

@amanj120 Don't forget to squash the commits.

[lentsius-bark]

I am so looking forward to this btw.

[akien-mga]

Thanks!

[fuzzypurplepixel]

Does this support Play Asset Delivery and/or Play Feature Delivery. I'm interested in separating parts of my game which are In app purchase access only.

[m4gr3d]

@fuzzypurplepixel This PR doesn't and it won't be available in Godot 3.2.4, but we're working on making it available in a future release.