Make NitzStateMachine testable

This commit does not modify behavior of NitzStateMachine.
It is a refactoring that separates NitzStateMachine from
various device concerns that would otherwise make unit
testing difficult.

There are three main categories of interaction from
NitzStateManager with other components:

1) External actions / querying of the NITZ state triggered
by ServiceStateTracker.

2) NitzStateManager interacting with the device's
time / time zone state and associated user settings. This
is now handled by TimeServiceHelper. In future this will
mostly be replaced by calls to a proposed TimeDetectionService
as part of the work to improve support for devices that have
multiple ways of detecting time / time zone. Time / time zone
detection being enabled will dependent on which component is
asking.

3) NitzStateManager interacting with global / shared device
state. i.e. reading and writing SystemProperties and reading
unmodifiable clocks. Abstracting these has clear
advantages for unit testing since it allows simulation of
various device states and avoids modification of the system
properties (which is not permitted, anyway).

This commit contains a single new test in NitzStateMachineTest
to demonstrate the purpose of this change and prove that
there is nothing preventing more extensive time / time zone
detection tests being written in future commits.

The only test in ServiceStateTrackerTest that touched on NITZ
logic has been modified to reflect the fact it delegates to a
(test stubbed) NitzStateMachine and no longer handles the NITZ
logic itself.

Testing:

To run individual tests:

atest FrameworksTelephonyTests:com.android.internal.telephony.NitzStateMachineTest \
    FrameworksTelephonyTests:com.android.internal.telephony.ServiceStateTrackerTest

To run all telephony tests:

atest FrameworksTelephonyTests

Bug: 63743683
Test: See above

Change-Id: Ie0be1bc4a2d6baf4860ed93902c697f3edfa4d44