Move init bootcharting onto its own thread.

This reduces the overhead when bootcharting is on (obviously), but also
removes the "do we need to do anything for bootcharting?" check in cases
where we're not bootcharting.

Bug: http://b/23478578
Bug: http://b/33450491
Test: rebooted with bootcharting on/off
Change-Id: Id8746b5023b17d7615679eba0bcd02aee048ef1a