bootstat: refine sys.boot.reason if device boot up from shipmode

- skip BootReasonStrToReason() if device boot up from shipmode
- skip BootReasonAddToHistory() if device boot up from shipmode
  and system_boot_reason is factory_reset or ota

Bug: 349723310
Test: verify sys.boot.reason work as expected if
      device boot up from shipmode
Change-Id: I4279deae8c14977c684b5b77035f16fb1b20ed71
Signed-off-by: Jason Chiu <jasoncschiu@google.com>