feat: register as Android default digital assistant (#757) (#758)

* feat: register as Android default digital assistant

* fix: assistant role request fallback + always-attempt click

* remove: assistant role request row from Settings

The system-level assistant registration stays (VoiceInteractionService
+ ROLE_ASSISTANT, set via system Settings -> Default apps), but the
in-app Settings row is dropped: RequestRoleActivity fails to resolve
the caller on a plain startActivity ('Package name cannot be null or
empty'), so the button was a dead end. Removes the row, the
AssistantRole helper, and the now-unused strings.

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/AndroidManifest.xml

@@ -29,6 +29,28 @@
             </intent-filter>
         </activity>
 
+        <service
+            android:name=".assistant.AssistantService"
+            android:permission="android.permission.BIND_VOICE_INTERACTION"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.service.voice.VoiceInteractionService" />
+            </intent-filter>
+            <meta-data
+                android:name="android.voice_interaction"
+                android:resource="@xml/voice_interaction_service" />
+        </service>
+
+        <service
+            android:name=".assistant.AssistantSessionService"
+            android:permission="android.permission.BIND_VOICE_INTERACTION"
+            android:exported="true" />
+
+        <service
+            android:name=".assistant.AssistantRecognitionService"
+            android:permission="android.permission.BIND_RECOGNITION_SERVICE"
+            android:exported="true" />
+
         <service
             android:name=".notification.ChatNotificationService"
             android:foregroundServiceType="remoteMessaging"