pbx_app: Update outdated pbx_exec channel snapshots.

pbx_app: Update outdated pbx_exec channel snapshots.

pbx_exec makes a channel snapshot before executing applications.
This doesn't cause an issue during normal dialplan execution
where pbx_exec is called over and over again in succession.
However, if pbx_exec is called "one off", e.g. using
ast_pbx_exec_application, then a channel snapshot never ends
up getting made after the executed application returns, and
inaccurate snapshot information will linger for a while, causing
"core show channels", etc. to show erroneous info.

This is fixed by manually making a channel snapshot at the end
of ast_pbx_exec_application, since we anticipate that pbx_exec
might not get called again immediately.

ASTERISK-30367 #close

Change-Id: I2a5131053aa9d11badbc0ef2ef40b1f83d0af086