AMI: Add AMI event to expose hook flash events

Although Asterisk can receive and propogate flash events, it currently
provides no mechanism for doing anything with them itself.

This AMI event allows flash events to be processed by Asterisk.
Additionally, AST_CONTROL_FLASH is included in a switch statement
in channel.c to avoid throwing a warning when we shouldn't.

ASTERISK-29380

Change-Id: Ie17ffe65086e0282c88542e38eed6a461ec79e81