Foreground Service for native screenshot capturing

[julioz]

Fixes https://github.com/mattprecious/telescope/issues/75

This branch was built on top of https://github.com/mattprecious/telescope/pull/76, to leverage the version bumps and also the crash reproducibility. Please consider merging that PR (it also means the diff for this one is smaller than it looks).

You can follow the commit history for easier understanding of the changes, but in summary:

• Adds a subclass of Service, updates the AndroidManifest to refer to it and adds the FOREGROUND_SERVICE permission
• Moves the logic for native screenshot capturing via the MediaProjection API behind a class I called NativeScreenshotCapturer, enabling reuse. This means:
  • Pre-Lollipop: Native capturing is disabled
  • Pre-Q: Native capture happens directly in TelescopeLayout
  • Q and above: Native capture happens on TelescopeProjectionService, the new foreground service
• Establishes a communication channel between the new Service and TelescopeLayout by using a ResultReceiver that serializes the listener calls back via its IPC API.

Open question: Since the foreground service requires a notification, and thus a notification channel, I hardcoded some values for simplicity (like notification channel name, service ID, notification ID, and didn't define a notification icon). Is this something users of the library absolutely care about, and if so, would it be worth providing a public API via telescope's configuration methods to define them?

[JakeWharton]

Why move the actual media projection interaction to the service? The requirement is for a foreground service to be present (thus a notification), but not that the work is actually happening in the service. All we should need is an empty service and its notification that can be started and stopped by the existing code that interacts with media projection.

[julioz]

but not that the work is actually happening in the service

This is news to me, I might have misread the documentation.

Thanks for the clarification, I'll send out another PR as soon as I have the time.