Notification handling and work manager tweaks

[bjester]

Summary

• Reverts #175
• Adds a new notification channel, separating the foreground service notification and the task notifications
• Updates the foreground service notification to be completely separate with static int ID
• Updates task notifications to use a static int ID and a tag matching the work request ID to maintain notifications for each
• Creates an App class to define configuration for WorkManager, including a fixed thread pool executor, and moves notification channel creation to it
• Disables automatic initialization of WorkManager so we only initialize it in our foreground service process
• Ensures we use RemoteWorkManager when queuing / checking work requests (ref)
• Updates worker to use tags for noting long-running tasks
• Creates Notifier interface, NotificationRef and NotificationBuilder classes for utility in handling notifications
• Updates our Worker implementation to function more like the standard Worker which opens a background thread for processing automatically (ref)
• Adds button to foreground service notification that takes user to settings so they can minimize it
• Adds ticker text to notifications used by accessibility services