[Thread] add missing permissions to ThreadNetworkDemoApp

The privileged permissions needs to be included in the allowlist to be
able to pre-install the ThreadNetworkDemoApp.

Otherwise RuntimeException like below will be thrown to stop the device
from booting up:

```
03-18 07:13:46.288  4920  4920 E AndroidRuntime: java.lang.IllegalStateException: Signature|privileged permissions not in privileged permission allowlist:
```

Bug: 330097284
Test: `lunch aosp_cf_x86_64_phone-trunk_staging-userdebug && m` and
verified that the demoapp can be started

Change-Id: Ia6876e2eff1cde837be9e69954b2c749525e9e3c
diff --git a/thread/demoapp/Android.bp b/thread/demoapp/Android.bp
index fcfd469..00f8090 100644
--- a/thread/demoapp/Android.bp
+++ b/thread/demoapp/Android.bp
@@ -34,7 +34,17 @@
     libs: [
         "framework-connectivity-t",
     ],
+    required: [
+        "privapp-permissions-com.android.threadnetwork.demoapp",
+    ],
     certificate: "platform",
     privileged: true,
     platform_apis: true,
 }
+
+prebuilt_etc {
+    name: "privapp-permissions-com.android.threadnetwork.demoapp",
+    src: "privapp-permissions-com.android.threadnetwork.demoapp.xml",
+    sub_dir: "permissions",
+    filename_from_src: true,
+}