Reland "Only allow timezone changes when done button is clicked"

This is a reland of ff7bc1fdb3eaf309893a4c8f1c79e0d26bcc9107

Original change's description:
> Only allow timezone changes when done button is clicked
> 
> Instead of changing the system timezone when a new value is selected on
> the list, it is only applied when the set date time dialog is closed.
> This is necessary to ensure that only parents can change time and
> timezone settings for children.
> 
> Bug: 921145
> Change-Id: If5f3460381fc60607741828e59912c669af05a9b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658858
> Reviewed-by: Aga Wronska <agawronska@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Henrique Grandinetti <hgrandinetti@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#670069}

Bug: 921145
Change-Id: Iea1a3557ea72e8637b20d92b6f3a7b1359220a66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665628
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Henrique Grandinetti <hgrandinetti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670544}