Validate them in ApiMessageTrait when the message is created, and again
in ApiMain before they're included in the header.
This also introduces an "api-warning" log channel, since "api" is too
spammy for real use, and converts a few existing things to use it.
Bug: T208926
Change-Id: Ib2d8bd4d4a5d58af76431835ba783c148de7792a
Depends-On: Iced44f2602d57eea9a2d15aee5b8c9a50092b49c
Depends-On: I5c2747f527c30ded7a614feb26f5777d901bd512
Depends-On: I9c9bd8f5309518fcbab7179fb71d209c005e5e64
