Add Debounce text to ACTION_NETWORK_COUNTRY_CHANGE

Explain that once a country signal is lost, the broadcast
will be delayed for some time until we are confident that
we have lost visibility to any country (unless we detect
a new country).

Bug: 136036974
Test: make offline-sdk-docs (docstring-only change)
Change-Id: I29536ca2d60968405bb476a97dfbda69487c0013