Refactoring: Support alt. network time source

Refactoring to support an alternative source of network time for passing
to the GNSS component. The new implementation will be submitted in a
follow-up.

NtpTimeHelper has been replaced by NetworkTimeHelper in
GnssLocationProvider. NetworkTimeHelper provides the stable interface
between GnssLocationProvider and the original / alternative impl for
what was NtpTimeHelper.

NtpTimeHelper has been renamed NtpNetworkTimeHelper.

These changes are not intended to change any behavior. There are some
minor changes between the interaction between GnssLocationProvider and
the NetworkTimeHelper class, but these are not expected to alter the
runtime behavior.

The NetworkTimeHelper.setPeriodicTimeInjectionMode() method touches
a pre-existing bug: The method name reflected the effect of the method,
which is the near-opposite of what the capability name would suggest.
This appears to be due to an accidental logic inversion, not by intent.
As can be seen in the changes for GnssLocationProvider: the
enablePeriodicTimeInjection() method was called when
mGnssNative.getCapabilities().hasOnDemandTime() was true. The existence
of bug 73893222 supports the fact that there is a long-standing bug
here. The intent with this commit is not to fix it or alter behavior,
just to make it more obvious, as it is unclear if the current behavior
is relied upon somewhere. Comments and field names have been improved to
try to clarify the actual behavior.

Bug: 73893222
Bug: 222295093
Test: atest services/robotests/src/com/android/server/location/gnss/NtpNetworkTimeHelperTest.java
Change-Id: I0b1ba43a55ff531df343c022650e3f5721dda7f1