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
diff --git a/private/file_contexts b/private/file_contexts
index 3955708..0ee01b8 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -697,6 +697,7 @@
 /metadata/gsi/ota(/.*)?   u:object_r:ota_metadata_file:s0
 /metadata/password_slots(/.*)?    u:object_r:password_slot_metadata_file:s0
 /metadata/ota(/.*)?       u:object_r:ota_metadata_file:s0
+/metadata/bootstat(/.*)?  u:object_r:metadata_bootstat_file:s0
 
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