getNetworkCountryIso API should return empty when signal is lost.

EmergencyNumberTracker provides shared emergency country iso in DSDS through getNetworkCountryIso API; it has been using last known country iso for caching emergency database when signal is lost; however, it should not provide the last known country iso when signal is lost through the getNetworkCountryIso API, which aims to return empty when signal is lost.

Test: atest
Bug: 179299344

Change-Id: Ia90b0210a6430bab6a2c4a6fa424e100c1e83e4d
Merged-In: Ia90b0210a6430bab6a2c4a6fa424e100c1e83e4d
(cherry picked from commit 558a738b)