Make the Emacs icon ``adaptive''

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.