Add a script to record Android boot time.

perfboot.py repeats the record of each event log during Android
boot specified times. By default, interval between measurements
is adjusted in such a way that CPUs are cooled down sufficiently
to avoid boot time slowdown caused by CPU thermal throttling.
This script also works around the issue of dropbox slowing down
boot time on userdebug build (http://b/20890386) by limiting
the number of files to be created by dropbox.
The result is output in a tab-separated value format.

BUG: 22207911
Change-Id: I0ddbac5d1c941efda87bc6db6388d8194d4bb3dd