UserspaceRebootLogger: query sys.boot.reason.last

Also did a small refactoring in computeOutcome() method to make it work
for cases when boot reason has a sub-reason, e.g.:
reboot,mount_userdata_failed,no_userdata_in_fstab.

Test: adb shell svc reboot userspace
Bug: 148767783
Change-Id: I13474d8a5be9382f56fdc8f17cf201666492e60d