Do not crash when duplication found in merged imsi list

For some unknown reason, the merged imsi list might contain
duplicated items. Deduplication for better error handling.

Test: NetworkPolicyManagerServiceTest
Bug: 243015487
Fix: 261225039
Change-Id: I69d332ee0acde6cdd7582d1aa18c82576f7090f2
Merged-In: I69d332ee0acde6cdd7582d1aa18c82576f7090f2
  (cherry-picked from aosp/2198759)