Disallow requesting networks with mutable NetworkCapabilities.

It's not clear what it means to request a network with a mutable
NetworkCapability like NET_CAPABILITY_VALIDATED or
NET_CAPABILITY_CAPTIVE_PORTAL.  Presently requesting such a network
would fail in a number of different ways:
1. The NetworkFactories would fail to match the request against their
   filter which doesn't include stateful NetworkCapabilities.
2. If the NetworkFactories did match, they'd bring up networks to try
   and satisfy the requests, but the networks would not have any
   mutable NetworkCapabilities initially so they'd be reaped.
Because of these problems it's safest to simply disallow these
requests.

Bug: 21343774
Change-Id: I56303242b81d39b370b8d5d1e32059bfcfc25949