Abort long-running benchmarks, report progress.

A typical storage device finishes the benchmark in under 10 seconds,
but some extremely slow devices can take minutes, resulting in a
confusing UX that looks like we've frozen.  Even worse, we keep
churning through all that I/O even though we know the device will
blow past our user-warning threshold.

So periodically check if we've timed out, and also use that to report
progress up into the Settings UI.

Test: manual
Bug: 62201209, 65639764, 67055204
Exempt-From-Owner-Approval: I wrote the original code.
Change-Id: Idd9d8c181651202d1434af879cff9355478a25c1