Command-line interface: NetworkTimeUpdateService

Initial command-line interface for NetworkTimeUpdateService for use in
tests.

To be joined by more commands as they are identified.

Bug: 213393821
Test: adb shell cmd network_time_update_service help
Test: adb shell cmd network_time_update_service clear_time
      && adb shell dumpsys network_time_update_service
Test: adb shell cmd network_time_update_service force_refresh
      && adb shell dumpsys network_time_update_service
Change-Id: I033764a7df9759e9fd4da0b0776f6535a574ff59