Unit tests of different ViewModels

Description

Includes various unit tests for viewmodels of vlc-android.

Motivation and Context

The code needs automated tests to analyse its stability. So, LibVLC needs to work without actual device and native libraries. The tests aren't done completely, but I wanted them to be reviewed along with the API changes I did.

How Has This Been Tested?

I did test it with actual APK, but it seems the previous medialibrary changes in upstream/master have broken it and so medialibrary doesn't work.

Screenshots / GIFs (if appropriate):

Types of changes

The implementation part was abstracted away for LibVLC to accomodate an on-demand request of implementations based on given interfaces. For that, AbstractFactoryManager design pattern is used.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which cleans up / improves existing functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING section of the README document.