bootstat: Do not allow unknown boot reasons to land in first field.

If we sniff an unknown boot reason from last kmsg, make sure it
has a "reboot," prefix.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: Ia1c401b8899d1f0c56bd4f5d8d2d19b7fc889a30