Add update_channel field to bootloader_message_ab.

The update_channel field is used to store the Omaha update channel
if update_engine is compiled with Omaha support.

We need it to be in misc to persist through factory reset.

Bug: 72332119
Test: mma
Change-Id: Ied4fecc6e78cc69d33a36ba4d101d675100f9d82