bootstat: introduce sys.boot.reason

Adding functionality to bootstat --record_boot_complete and
--record_boot_reason to initialize sys.boot.reason, the canonical
system boot reason.

Filter out ro.boot.bootreason oem noise into sys.boot.reason.  Add
heuristics to determine what the boot reason is, when otherwise would
be defaulting to the blunt and relatively devoid of detail catch-all
reboot reasons ("reboot", "shutdown", "cold", "warm", "hard").

boot_reason_test.sh is also a compliance test.

Test: boot_reason_test.sh all
Bug: 63736262
Change-Id: Ic9a42cccbcfc89a5c0e081ba66d577a97c8c8c76