Add support for multiple NTP servers

Add support for multiple NTP servers in NtpTrustedTime. The algorithm to
select the server is documented in comments in that class. Tests have
been updated accordingly.

This commit changes config.xml, removing (string) config_ntpServer and
replacing it with (string-array) config_ntpServers. It also modifies
command line and settings behavior.

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: adb shell settings put global ntp_server "ntp://time.google.com\|ntp://time.android.com"
Test: Inspection: adb shell dumpsys network_time_update_service
Test: Inspection: adb shell dumpsys time_detector
Change-Id: I5e19a4d82c9771448940b9f1db1689f21508667b