Tell netd about local networks

This patch does the following :
• When a network is local but the system doesn't support it,
  throw in registerNetworkAgent.
• When a network is local, tell netd about it.
• Add tests for the above, as well as a test for callbacks
  being sent correctly when the caps are set and not when
  they aren't and that the new keep connected flag is
  respected.

Telling netd about the network being local has it add two
routes that do not exist if the network is not local :
• One at PRIORITY_LOCAL_NETWORK matching unmarked sockets
  and looking up the table for the interface.
20000: from all fwmark 0x0/0x10000 lookup xxxx
• One at PRIORITY_EXPLICIT_NETWORK matching explicitly on
  network 99, so that dnsmasq traffic and any OEM traffic
  marked for network 99 flowing there.
16000: from all fwmark 0x10063/0x1ffff iif lo lookup xxxx
IMPLICIT_NETWORK and EXPLICIT_NETWORK rules are installed
by connectivity service as a matter of course whether the
network is local or not.

See commit If8729fc6f3716a580c936584b851bc38000b5de5 for
implementation details of this mechanic.

There is no need to implement anything in particular for the
new connected reason, as the current implementation will
already keep it connected. A new test makes sure of that.

Test: FrameworksNetTests
      CtsNetTestCases
      FrameworksNetIntegrationTests
      NetworkStackTests
      NetworkStaticLibTests
      TetheringTests
      MtsTetheringTestLatestSdk
      TetheringIntegrationTests
Change-Id: I6fb7dfe4c232eea8cac9ac268897ddb36bb794d1