Demo: Add legacy external storage permission for intents.

Without this permission, files in `/sdcard` or other directories cannot be read by the demo, as they're not in the transformer demo's scoped storage container.

For more information, see https://developer.android.com/training/data-storage/use-cases

Tested by uninstalling and re-installing the demo app, granting permission by
starting a transformation, and launching an intent using a local file in `/sdcard`
to start another transformation. Without this CL, this threw an error, and with
this CL it succeeded.

PiperOrigin-RevId: 466399023

demos/transformer/src/main/AndroidManifest.xml

@@ -29,6 +29,7 @@
       android:label="@string/app_name"
       android:theme="@style/Theme.AppCompat"
       android:taskAffinity=""
+      android:requestLegacyExternalStorage="true"
       tools:targetApi="29">
     <activity android:name=".ConfigurationActivity"
         android:configChanges="keyboard|keyboardHidden|orientation|screenSize|screenLayout|smallestScreenSize|uiMode"