bootstat: record reboot reason history in persist.sys.boot.reason.history

Retain the last four reboot reasons from latest to oldest, adding a
detail of UTC epoch time for each.  Beef up boot_reason_test.sh to
deal with multi-line content.

Test: boot_reason_test.sh
Bug: 110925971
Change-Id: Ie2600434b95a885693f005a363ee38d9eba18ee7