Move useExternalPlayer and getPlaybackActivityClass functions to new PlaybackLauncher interface

[nielsvanvelzen]

This change is to make sure development for the new playback code can happen in the master branch while still being able to release updates with the old code. It does this by adding a new interface "PlaybackLauncher" that is used when launching the playback activity and a toggle for which implementation to use.

When the initial playback module is added this will launch an activity that reads the data from the current playback code (item to play etc.) and converts it to the new playback module format before launching the new playback module activity. This is a bit hacky but good enough for development.

The implementation toggle only works on debug builds of the app and will never show up on releases.

Changes

• Move useExternalPlayer and getPlaybackActivityClass functions to new PlaybackLauncher interface
• Implement PlaybackLauncher interface with old code
• "Implement" PlaybackLauncher interface for new playback code
• Add toggle to enable new playback module

Issues

Part of #1057