refactor: Restructure ViewMediaActivity and related fragments

[nikclayton]

Highlights:

• Implement fragment transitions for video to improve the UX, video won't start playing until the transition completes
• Remove rxJava
• Move duplicate code in to base classes

Details:

MediaActionsListener:

• Move to ViewMediaFragment as it's used by both subclasses
• Remove need for separate VideoActionsListener
• Rename methods to better reflect their purpose and improve readability

ViewMediaFragment:

• Move duplicated code from ViewImageFragment and ViewVideoFragment
• Rewrite code that handles fragment transitions to use a CompleteableDeferred instead of BehaviorSubject (removes rxJava).
• Rename methods and properties to better reflect their purpose and improve readability
• Add extra comments

ViewImageFragment:

• Rewrite code that handles fragment transitions to use a CompleteableDeferred instead of BehaviorSubject (removes rxJava).

ViewVideoFragment:

• Implement fragment transitions for video to improve the UX, video won't start playing until the transition completes
• Manage toolbar visibility with a coroutine instead of a handler
• Add extra comments

ViewMediaActivity:

• Rename properties to better reflect their purpose and improve readability
• Add extra comments

ImagePagerAdapter:

• Rename properties to better reflect their purpose and improve readability
• Add extra comments