Support disconnecting while trying to connect.

Currently, once a NetworkAgent has decided to connect, there's
no way to disconnect without first connecting and having
ConnectivityService tear down the connection. This is suboptimal
because it causes the transport to keep retrying even if it knows
that it will not be able to connect.

Instead, allow the transport to abort a connection request that's
in progress, as long as the agent is not yet registered with
ConnectivityService.

Also add locking to evalScores. evalScores should already have
been taking a lock, because it accesses member variables that are
also accessed by the send*methods.

Bug: 15295359
Change-Id: I913c341bdfc50be9c23b632399f53168e754c1c0