ami: Improve substring parsing for disabled events.

ASTERISK_29853 added the ability to selectively disable
AMI events on a global basis, but the logic for this uses
strstr which means that events with names which are the prefix
of another event, if disabled, could disable those events as
well.

Instead, we account for this possibility to prevent this
undesired behavior from occuring.

ASTERISK_29853

Change-Id: Icccd1872602889806740971e4adf932f92466959