aggregate the items of network selection list

According to bug comment#44, modem only report one PLMN per RAT
for different cells reported for the same PLMN/RAT combination,
UI affregate those items.
Bug: 173387374
Test: atest NetworkSelectSettingsTest

Change-Id: I155abc8dda7c8ec42cf4fd677bd2aa99d1033ad1
Merged-In: I8e05b9f6784bad7f0de4e79cc6e707749745f0c1