NetworkPolicyManager: Annotate test

Annotate test introduced in 57e3d310
with @test. Fixes the error-prone build.

Bug: 28791717
Test: m javac-check RUN_ERROR_PRONE=true
Test: m -j32 FrameworksServicesTests && adb install -r -g ${ANDROID_PRODUCT_OUT}/data/app/FrameworksServicesTests/FrameworksServicesTests.apk && adb shell am instrument -e class "com.android.server.NetworkPolicyManagerServiceTest" -w "com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner"
Change-Id: I03c1cde093be91fe52d83f2f2ffeb69a126a27d5