Compile NetworkStack resources only once

The current build files would compile resources both for the
android_library and the android_app build rules, since the android_app
build rule resource_dirs default to ["res"]. Set resource_dirs to [] for
these build rules so that resources are only inherited from the
android_library.

Also refactor the build file using java_defaults to avoid repeating
configuration.

Test: flashed, boots, WiFi working
Bug: 127908503
Change-Id: Ie51dd1bd1a8ee5145e3f1b95beb1de59665b4c51