bootstat: clear persist.sys.boot.reason once read

To ensure a surprise reboot does not take the last boot reason on
face value especially if coming from more than one boot sessions ago.
We shift and clear the value from persist.sys.boot.reason to
sys.boot.reason.last and establish a correct last reboot reason in
the canonical sys.boot.reason property.

This effectively deprecates persist.sys.boot.reason as an API.  They
should have been using sys.boot.reason instead for a correctly
determined reasoning.

Test: boot_reason_test.sh
Bug: 86671991
Merged-In: If85750704445088fd62978679ab3a30744c46abb
Change-Id: If85750704445088fd62978679ab3a30744c46abb