Migrate Session class from Java to Kotlin

[johnjohndoe]

Current state

• The Session class in the app module is written in Java.
• As the central data holder in the UI layer its usage is wide-spread and critical to all app features.
• Fields of the class are public, hence can be modified from external.
• The class contains static functions which are contextual related to the Session class.
• Not all field are part of the equals and hashCode methods.

Target state

• The Session class in the app module is written in Kotlin as a data class.
• Properties of the class are initialized at construction time and cannot be modified. This makes Session instances immutable.
• Static functions are moved into separate well named files/classes.
• Not all field are part of the equals and hashCode functions implicit data class. This might be addressed in subsequent issues / merge requests.

Motivation

• Mutable Session instances which miss the benefits of a Kotlin data class make development uncomfortable.
• Writing tests with Session Java instances is cumbersome and painful. Writing test should be fun!

Approach

• The migration happens step-wise to facilitate small, easy to comprehend commits.
• Clean-up happens before larger, and functional changes.
• Tests are added before a change to prevent introducing bugs.

Related

• Complete switch to Kotlin #448