Merge "bootstat: Followup cleanups from the initial bootstat checkin."

    $ bootstat -l

bootstat logs all boot events recorded using the `-r` option to the EventLog

using the Tron histogram. On GMS devices these logs are uploaded via Clearcut

for aggregation and analysis.

using the Tron histogram. These logs may be uploaded by interested parties

for aggregation and analysis of boot time across different devices and

versions.

## Printing boot events ##