Add more test coverage for requests and callbacks.

1. Support multiple callbacks in TestNetworkCallback. This is
   necessary to test situations where multiple callbacks are
   generated by the same event (e.g., CALLBACK_LOSING on cell
   with CALLBACK_AVAILABLE on wifi when wifi connects), which is
   necessary to test callback order. So far this has not been
   covered because all callback testing was using per-network
   callbacks.
2. Add a benchmark test for registering NetworkRequests and for
   sending onAvailable and onLosing callbacks.

Bug: 23113288
Change-Id: Ib69373358ad766ab1bd989e864a5a51ef762c73c