Update settings for Date / Time / Zone

Update SettingsProtoDumpUtil / secure.proto to reference the new (user
scoped) location_time_zone_detection_enabled setting that has been added
for S.  This has been added in a "DateTime" message to reflect the
SettingsUI structure.

For consistency with secure.proto and the settings strings, global.proto
and the associated SettingsProtoDumpUtil code has updated to rename the
"Auto.time" and "Auto.time_zone" proto fields to "DateTime.auto_time"
and "DateTime.auto_time_zone". This is a binary compatible change as
the field IDs have not been changed.

Note: The "time_12_24" and "date_format" settings, which are user-scoped
settings from "Settings.System" have been left for a future release
besides some minor docs improvements. See bug 185884644.

This change also improve the docs for settings that are associated with
date and time.

Bug: 151304765
Bug: 185884644
Test: build / treehugger
Change-Id: If2daf3f530bd1add7a0a5cc3260221ea3ad8952d