fix(android): support cdvfile assets for custom scheme

[erisu]

Platforms affected

android

Motivation and Context

cdvfile:// protocol is no longer supported and the assets file system was not working with recent changes to support the new cdvfile url pattern.

This PR is to support the assets file sytem

Description

Updated the WebViewAssetLoader.PathHandler to include assets

E.g. https://localhost/__cdvfile_assets__/ will map to app's assets /android_asset/

Testing

• cordova build
• cordova run
• test specs

Checklist

• I've run the tests to see all new and existing tests pass
• I updated automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[NiklasMerz]

LGTM