bootstat: usb and rtc refinement

For all known cases, if usb is present in the bootloader reason,
then it is actually reporting a cold,charger canonical boot reason.
This signifies that the device was powered down, and was woken
up by the charger being connected.

For all known cases, if rtc is present in the bootloader reason,
then it is actually reporting a cold,rtc canonical boot reason.
This signifies that the device was powered down, and was woken
up by the rtc clock.

Test: boot_reason_test.sh
Bug: 74595769
Bug: 63736262
Change-Id: I04bede0b7ccaa1b859943f7def93521a8f7b25c6