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)
Merged-In: I29536ca2d60968405bb476a97dfbda69487c0013
Change-Id: I29536ca2d60968405bb476a97dfbda69487c0013
(cherry picked from commit 3bdc7e4c62)

telephony/java/android/telephony/TelephonyManager.java

@@ -11125,6 +11125,8 @@ public class TelephonyManager {
      * The {@link #EXTRA_NETWORK_COUNTRY} extra indicates the country code of the current
      * network returned by {@link #getNetworkCountryIso()}.
      *
+     * <p>There may be a delay of several minutes before reporting that no country is detected.
+     *
      * @see #EXTRA_NETWORK_COUNTRY
      * @see #getNetworkCountryIso()
      */