Add time detector network behavior

Add methods to enable the latest network time signal to be retrieved and
cleared. Also add shell command support for their use in tests.

getLatestNetworkSuggestion() already existed for internal use and has
now been exposed.

clearNetworkTime() / clearLatestNetworkSuggestion() have been added for
use in a future commit.

Bug: 222295093
Test: build / boot / treehugger
Test: atest services/tests/servicestests/src/com/android/server/timedetector/TimeDetectorStrategyImplTest.java
Change-Id: I9ffc00b6f2f5a12cbe3e20fc5be544fd2aedee37