bootstat: Add support for logging the boot_reason metric.

This value is read from the ro.boot.bootreason system property.

Bug: 21724738
Change-Id: I43bef3d85ba9c8d87669a91da1aa675d9a86e348
(cherry picked from commit a4a1a4ac8556fd9550d5b6c916546519b3641fb0)
7 files changed