🛡️ Guard Flutter Android app by disallow task affinity by default (#144018)

Partial resolution for #63559

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packages/flutter_tools/templates/app_shared/android.tmpl/app/src/main/AndroidManifest.xml.tmpl

@@ -7,6 +7,7 @@
             android:name=".MainActivity"
             android:exported="true"
             android:launchMode="singleTop"
+            android:taskAffinity=""
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
             android:hardwareAccelerated="true"