Log the time it takes netd to start up.

Currently on, bullhead-eng, I see:

01-21 14:59:26.174 21421 21421 I Netd    : Netd started in 2432ms

Test: restarted netd and observed log message.
Bug: 32323979
Bug: 33279878
Change-Id: I7195d06d7ed1a09858185555f60b07e5bfe306ed