Background process

[a-mabe]

Implements #58

• Uses the background timer package
• The timer continues to run even if OpenHIIT is not the currently active application

TODO

• Set the iOS icon back to normal
• Debug the process randomly starting on Android
• When playing a workout, display the exercise name instead of "Work" above the countdown
• Code cleanup
• Point app to the main branch of the background process package
• Only use sound_pool for audio