Avoid over logging preference changes.

When preference screen is created all prefence goes through
SharedPreferenceLogger once. We don't want to log it as preference
change because it's just initial display. Subsequent preference logger
calls should result in logging.

Bug: 30914916
Test: make RunSettingsRoboTests
Change-Id: Icaee9137c55555f4db7839ff0381a13ccd420190