bootstat: refine boot reasons

Received some clarity as to some of the boot reasons.

List of boot reasons and new translations to Canonical boot reason:

- "power_key" -> "cold,powerkey" (existing)
- "usb" -> "cold,charger" (existing)
- "rtc" -> "cold,rtc" (existing)
- "wdt" -> "reboot" (changed)
- "wdt_by_pass_pwk" -> "warm" (changed)
- "tool_by_pass_pwd" -> "reboot,tool" (changed)
- "2sec_reboot" -> "cold,rtc,2sec" (changed)
- "unknown" -> "reboot,unknown" (existing)
- "kernel_panic" (existing)
- "reboot" (existing)
- "watchdog" (existing)

Add the new string to enums for the new Boot Reason.

Test: boot_reason_test.sh (on affected device)
Bug: 74595769
Bug: 63736262
Change-Id: Iecedc3b1f7c47f26d0c77b1f316f745c6d2c1256