bridge-infinite-wait: Update examples to have a periodic playback

This updates the examples in bridge-infinite-wait to be a bit more interesting. When comparing
them against bridge-hold, I realized that there wasn't much more we were doing other than
a bit of bridge management.

This patch adds a periodic announcement done by playing into the bridge using the /play
operation. This requires some management of Music on Hold (due to an annoying quirk in
Asterisk that should probably be fixed). It also makes the 'your call is important to us' message
both more interesting in the example, as well as more annoying.
That's also a good idea for the ChannelLeftBridge handler. When attached to a channel object, it should only fire one time in the context of this application - once they've left the holding bridge, they'd have to re-enter the application, which would entail a new event handler.