bridge-infinite-wait/example.py: Re-tool Python example

This updates the Python example for bridge-infinite-wait in the following ways:
(1) It adds an announcement that occurs periodically
(2) It removes one of the channel left bridge handlers. The only time we need
    to handle this is from the bridge itself.
(3) We now toggle MoH on/off the bridge surrounding the announcements. This is
    to work around a limitation in Asterisk, where we don't currently pause
    and restart MoH when an announcement is triggered.