Make the Emacs icon ``adaptive''

* java/AndroidManifest.xml.in (EmacsApplication): Set icon to
@mipmap/emacs_icon.

* java/org/gnu/emacs/EmacsService.java (onStartCommand): Pick a
better name for the persistent notification channel.

* java/res/drawable/emacs_background.xml:

* java/res/drawable/emacs_foreground.xml:

* java/res/mipmap/emacs_icon.png:

* java/res/mipmap-v26/emacs_icon.xml: New files, comprising an
``adaptive icon'' required by some Android launchers.

java/org/gnu/emacs/EmacsService.java

@@ -171,7 +171,7 @@ public final class EmacsService extends Service
 		     + " may disable it if you want;"
 		     + " see (emacs)Android Environment.");
 	channel
-	  = new NotificationChannel ("emacs", "Emacs persistent notification",
+	  = new NotificationChannel ("emacs", "Emacs Background Service",
 				     NotificationManager.IMPORTANCE_DEFAULT);
 	manager.createNotificationChannel (channel);
 	notification = (new Notification.Builder (this, "emacs")