Add startDestination arg to SpaActivity

To let other activity open Spa pages.

Bug: 235727273
Test: Manual with Gallery App
Change-Id: I33b391813c0f40fdca6a52d7a9fd3114b2eb89be
diff --git a/packages/SettingsLib/Spa/gallery/AndroidManifest.xml b/packages/SettingsLib/Spa/gallery/AndroidManifest.xml
index b5be7cd..2229986 100644
--- a/packages/SettingsLib/Spa/gallery/AndroidManifest.xml
+++ b/packages/SettingsLib/Spa/gallery/AndroidManifest.xml
@@ -22,7 +22,7 @@
         android:label="@string/app_label"
         android:supportsRtl="true">
         <activity
-            android:name="com.android.settingslib.spa.gallery.MainActivity"
+            android:name=".MainActivity"
             android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />