Add a setting to support for NITZ retention

Add a setting to support for NITZ retention, see the comments for
details. The comments for related settings have also been updated to
better reflect their meaning / improve consistency with the new setting.

Bug: 204415203
Test: See frameworks/opt/telephony/ change
Change-Id: Idb21dc9ea7a1ad5cbb7a8bd238ab9a71174ee1ff