hello_xr: Export built-in NativeActivity

If exported attribute is true, the app can be launched by other apps on latest Android platform.
See https://developer.android.com/guide/topics/manifest/activity-element.

Signed-off-by: utzcoz <[email protected]>

Author: utzcoz

src/tests/hello_xr/AndroidManifest.xml

@@ -46,6 +46,7 @@
       android:resizeableActivity="false"
       android:screenOrientation="landscape"
       android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen"
+      android:exported="true"
       tools:ignore="NonResizeableActivity">
       <!-- Tell NativeActivity the name of the .so -->
       <meta-data