Add ability to override subscriber capabilities.

In a future set of CLs, NPMS will offer to override a handful of
capabilities on a per-subId basis.  Define a no-op version of the
interface to make it easier to add new methods in the future.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.
Test: bit FrameworksTelephonyTests:com.android.internal.telephony.dataconnection.DataConnectionTest
Bug: 64133169
Change-Id: I03dfd98463861f0338c4174e8d8a88c300ea5b55