Further refactoring for multiple NTP servers

Further refactoring to support adding multiple NTP servers in a later
commit. The logic for multiple servers will be added to forceRefresh()
later.

This commit also contains some refactoring to support improved testing
of NtpTrustedTime.forceRefresh(). Tests for the existing behavior have
been added here.

As part of this change, SntpClient / NtpTrustedTime /
NetworkTimeUpdateService have also been extended to pipe information
about the IP address of the NTP server used to obtain the time back to
the NetworkTimeUpdateService for improved logging / debugging support.

Bug: 223365217
Test: atest frameworks/base/core/tests/coretests/src/android/util/NtpTrustedTimeTest.java
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/timedetector/
Test: atest cts/tests/tests/os/src/android/os/cts/SystemClockSntpTest.java
Test: Inspection: adb shell dumpsys network_time_update_service
Test: Inspection: adb shell dumpsys time_detector
Change-Id: Ia0f13e2c8cbd102d677797b745414d8ab5fa1eba