Introduce an UpdateService for android.

Very simple so far. This schedules an alarm to fire once a day, kicking
off a service that downloads a new app.skyx from a hardcoded URL. The
new skyx replaces the current one.

examples/fitness/apk/AndroidManifest.xml

@@ -15,5 +15,9 @@
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
+        <service
+            android:name="org.domokit.sky.shell.UpdateService"
+            android:exported="false"
+            android:process=":remote"/>
     </application>
- </manifest>
+ </manifest>
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