Better handling of NTP-based clocks.

Now that we have a nice Clock abstraction, we can use it to represent
a clock backed by an NTP fix.  (This makes testing logic much easier
to write.)

We now rely completely on NetworkTimeUpdateService to keep our NTP
fix up to date, instead of trying to refresh in the middle of
critical paths which could trigger random ANRs.

Add internal FallbackClock to make it easier to handle missing NTP
fixes.  Add internal SimpleClock to let implementers focus on single
millis() method.

Test: bit FrameworksNetTests:com.android.server.net.NetworkStatsServiceTest
Test: bit FrameworksServicesTests:com.android.server.NetworkPolicyManagerServiceTest
Bug: 69714690, 72320957
Change-Id: Ic32cdcbe093d08b73b0e4b23d6910b23ea8e1968
Exempt-From-Owner-Approval: approved in previous PS