bootstat: enhance last reboot reason property with file backing

Helps with support of recovery and rollback boot reason history, by
also using /metadata/bootstat/persist.sys.boot.reason to file the
reboot reason.  For now, label this file metadata_bootstat_file.

Test: manual
Bug: 129007837
Change-Id: Id1d21c404067414847bef14a0c43f70cafe1a3e2
8 files changed