Declare to work on mobile and desktop devices

As per http://adrienplazas.com/blog/2020/10/15/specify-form-factors-in-your-librem-5-apps.html

briar-gtk/data/app.briar.gtk.desktop.in

@@ -9,3 +9,4 @@ Terminal=false
 Type=Application
 Categories=GTK;Network;Chat;InstantMessaging;
 X-GNOME-UsesNotifications=true
+X-Purism-FormFactor=Workstation;Mobile;

briar-gtk/data/app.briar.gtk.metainfo.xml.in

@@ -62,6 +62,11 @@
     <binary>briar-gtk</binary>
   </provides>
 
+  <custom>
+    <value key="Purism::form_factor">workstation</value>
+    <value key="Purism::form_factor">mobile</value>
+  </custom>
+
   <releases>
     <release version="0.1.0-beta2" date="2020-10-10">
       <description>