Fix performance degradation from BootSequence atom

(partial cherry pick from commit fe3e762b)

Adding the boot sequence reported atom in ag/3518079 caused the duration
of bootstat to increase, as seen in b/72864061. I isolated the cause
down to calling BootReasonStrToReason. However, this function also gets
called in ReportBootReason, so I created another function that does the
parsing and sets the system boot reason property, and made
RecordBootReason and statsd logging get that property.

Bug: 72864061
Test: rebooted phone, verified boot events were received in adb shell
logcat -b stats and verified adb shell bootstat -p printed correct
values. Ran timing tests as well on walleye with 20 boots: before this
change, the average was ~150-160ms. After, it was ~80ms.
Change-Id: I92dbc9880328835647be7d9d50c7861b42f27bdb
Merged-In: I92dbc9880328835647be7d9d50c7861b42f27bdb