Reduce the visibility of logging to statsd to package-private

This change makes CTLogger into a public interface and reduces the
visibility of the actual implementation to package-private, as it was only made
public to allow for testing via mocking.

Flag: com.android.net.ct.flags.certificate_transparency_service
Bug: 378626065
Test: atest NetworkSecurityUnitTests
Change-Id: Ifeab50244219396ba3d3fc7f87b8d6e31ff726c9
4 files changed