dumpstate: modify logcat timeout to compensate for storage speed

Discover the logcat buffer size, divide by the worst storage
speed in kb/s, and multiply by ten to estimate the timeout in
ms for each named buffer. If that value is less than 20 seconds,
then select 20 seconds. Apply this to each logcat request.

Bug: 21555259
Change-Id: Id36aebff83cb06e94350e4a84fa266b66a8444fc