Fix getStyle returns null after adding a source

[Chaoba]

PRs must be submitted under the terms of our Contributor License Agreement CLA.
Fixes: #634
Fixes: #583

Pull request checklist:

• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Add example if relevant.
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-android changelog: <changelog>Fix getStyle returns null after adding a source</changelog>.

Summary of changes

onStyleLoaded will only be triggered once for one style, so if users add a new source to style after style loaded, onStyleLoaded will never be invoked. And during the loading time for this new source, getStyle will return null because style.isStyleLoaded will be false.
This pr add a property styleLoaded to record wheter the style is loaded or not no matter how many sources or layers are added after style loaded.

User impact (optional)

[alexander-kulikovskii]

should it be with private setter?
because in Java internal converted into public

[Chaoba]

We need set it in test cases.

[alexander-kulikovskii]

maybe use annotation VisibleForTesting ?

[kiryldz]

@Chaoba please link an issue that gets fixed by this PR.
Also we should have a new test for it that was failing before but should pass now.

[pengdev]

This PR reverts @tobrun's PR in #332, where we used native isStyleLoaded instead of the platform side cached value. Would be good to get a clear understanding of the issue we experienced and check with gl-native team if we want to modify the behaviour of isStyleLoaded on the native side.

[pengdev]

Could be related to #614 as well

[pengdev]

@Chaoba could you build a snapshot using this branch?

[tobrun]

check with gl-native team if we want to modify the behaviour of isStyleLoaded on the native side.

This was already confirmed in the annotation related ticket that it's expected to return false when something new is loading. This is conform to gl-js implemenation. For our own internal style logic, we need to roll our own setup as proposed here.