[thread] loose the restrictions on ThreadNetworkManager APIs

1. We don't need to assert that ThreadNetworkManager is always null
   when the Thread hardware features are not present. There can be more
   useful Thread features which don't require the hardware
2. The getAllThreadNetworkControllers() API has no documentation says
   the returned list is guaranteed not empty.

Change-Id: Ie77110067ff670053a2125b30b2791b9f54b2582
1 file changed