Fix crash due to missing access token; delete CredentialsManager.kt

[Chaoba]

PRs must be submitted under the terms of our Contributor License Agreement CLA.
Fixes: < Link to related issues that will be fixed by this pull request, if they exist >

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 crash due to missing access token; delete CredentialsManager.kt</changelog>.

Summary of changes

While users only set token in attr of MapView, a MapboxConfigurationException will be thrown as we can't find token from resource file and default value. This pr applies the token from attr to fix it.

User impact (optional)

[pengdev]

Also thinking the ResourceOptionsManager stores the static default ResourceOptions, and access token is part of the ResourceOptions, so feels like CredentialsManager is redundant, shall we discuss whether we should remove the CredentialsManager?

[kiryldz]

I'll drop 🚢 here as changes look ok to me and they fix a concrete bug. However perhaps we want to consider some alignment with iOS beforehand.

[pengdev]

LGTM with nits