notification sound stutters on Android 8.0 and 8.1

[1fish2]

The app's notification sound frequently stutters and restarts midstream, but only on Android 8.0 and 8.1. This happens in the emulator and actual hardware.

Aha! After investigating, this seems to happen when GCMMessageHandler#handleMessage builds a notification and then a summary notification.

So starting from no TBA notifications in the shade, the first one will sound right and (if nobody dismisses it) the next one will play stuttered.

[1fish2]

The fix is to call builder.setGroupAlertBehavior(NotificationCompat.GROUP_ALERT_SUMMARY) before building a notification that will immediately become part of a group summary. But this new method is in supportLibVersion 26.1.0 and compileSdkVersion 26, and calling it requires restructuring the code to push a NotificationCompat.Builder around rather than a built Notification.

I'll add it to PR #861

[1fish2]

PR #861 ought to fix this bug.