feat(android-agent): Meilenstein 2 — sehen (Screenshot + ui_dump)

Screenshot via MediaProjection (ScreenCapturer, gleicher {format,bytes,base64}-
Vertrag wie der Desktop-Agent -> host_screenshot, inkl. Vision). UI-Baum via
AriaAccessibilityService (nur lesend) -> neues Brain-Tool host_ui_dump. Freigabe
einmalig in der App: 'Bildschirm-Zugriff erlauben' + 'Bedienungshilfe oeffnen'.
caps = [info, screenshot, ui_dump]. targetSdk 33 -> keine mediaProjection-FGS-
Typpflicht.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host-agent/android/app/src/main/AndroidManifest.xml

@@ -39,5 +39,18 @@
                 <action android:name="android.intent.action.BOOT_COMPLETED" />
             </intent-filter>
         </receiver>
+
+        <service
+            android:name=".AriaAccessibilityService"
+            android:exported="false"
+            android:label="ARIA Host-Agent"
+            android:permission="android.permission.BIND_ACCESSIBILITY_SERVICE">
+            <intent-filter>
+                <action android:name="android.accessibilityservice.AccessibilityService" />
+            </intent-filter>
+            <meta-data
+                android:name="android.accessibilityservice"
+                android:resource="@xml/accessibility_config" />
+        </service>
     </application>
 </manifest>