android: battery saving instructions screen as in the Final 2026 mockup

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -79,6 +79,9 @@
                 <action android:name="android.service.quicksettings.action.QS_TILE_PREFERENCES" />
             </intent-filter>
         </activity>
+        <activity
+            android:name=".ui.BatteryActivity"
+            android:exported="false" />
         <activity
             android:name=".ui.VpnProfileControlActivity"
             android:theme="@style/TransparentActivity"