Add NetworkStackTests to device-tests suite

It's needed for the test to run in a Test Mapping suite.

Bug: 123544631
Test: local build
Change-Id: I0678a3e8f9beed7c637c1b11594369aa5d3c87bf

packages/NetworkStack/tests/Android.bp

@@ -18,6 +18,7 @@ android_test {
     name: "NetworkStackTests",
     certificate: "platform",
     srcs: ["src/**/*.java"],
+    test_suites: ["device-tests"],
     resource_dirs: ["res"],
     static_libs: [
         "android-support-test",