Fix: keep isForegroundService flag in sync with service state

[mhelder]

This closes #444.

The local flag isForegroundService is currently only set to false when the notification is cancelled:

uamp/common/src/main/java/com/example/android/uamp/media/MusicService.kt

Lines 592 to 596 in 8dda0e8

	override fun onNotificationCancelled(notificationId: Int, dismissedByUser: Boolean) {
	stopForeground(true)
	isForegroundService = false
	stopSelf()
	}

There is however another scenario that will stop the service from running in the foreground: when the player's playback state changes to paused (playWhenReady == false)

uamp/common/src/main/java/com/example/android/uamp/media/MusicService.kt

Lines 615 to 621 in 8dda0e8

	if (!playWhenReady) {
	// If playback is paused we remove the foreground state which allows the
	// notification to be dismissed. An alternative would be to provide a
	// "close" button in the notification which stops playback and clears
	// the notification.
	stopForeground(false)
	}

In this last scenario, isForegroundService is not set to false, which prevents it from being promoted to the foreground again when a notification update is posted:

uamp/common/src/main/java/com/example/android/uamp/media/MusicService.kt

Lines 581 to 590 in 8dda0e8

	if (ongoing && !isForegroundService) {
	ContextCompat.startForegroundService(
	applicationContext,
	Intent(applicationContext, this@MusicService.javaClass)
	)
	startForeground(notificationId, notification)
	isForegroundService = true
	}
	}

This PR adds that missing statement, allowing the service to be promoted to the foreground again when playback resumes.

For a more extensive description of the bug, see issue description.

[nic0lette]

Thank you!