Improve logging in bootstat.

This should help in debugging issues related to the mismatch between
actual last reboot reason property and the expected one (see attached
bug).

Test: adb reboot
Test: adb logcat | grep bootstat
Bug: 152900920
Change-Id: I085cf1fb80a30389fd3ba821d40b6111a3294d95
Merged-In: I085cf1fb80a30389fd3ba821d40b6111a3294d95
(cherry picked from commit 49062f3b)