SimpleDeviceConfig: Install to system_ext

This isn't a core or essential system component.
diff --git a/Android.bp b/Android.bp
index 7f66b30..8148ddf 100644
--- a/Android.bp
+++ b/Android.bp
@@ -17,6 +17,7 @@
     srcs: ["src/**/*.java"],
     resource_dirs: ["res"],
     platform_apis: true,
+    system_ext_specific: true,
     certificate: "platform",
     privileged: true,
     manifest: "AndroidManifest.xml",