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
1bc034cb87