Access networks manager changes

1. Blocked invalid access networks types. UNKNOWN
   is not a valid access network type.
2. Empty list can clear the previous suggested
   access network types.

Fix: 226267529
Test: atest AccessNetworksManagerTest
Merged-In: I391d068697dc04e0501bda9fdb5508b0ddea90e4
Change-Id: I391d068697dc04e0501bda9fdb5508b0ddea90e4