Implement auto 12/24h setting toggle.

This implements an explicit toggle to enable/disable automatic 12h/24h
time formatting detection based on the current locale.

Previously automatic detection was the norm on a freshly wiped device,
but could never be re-enabled once either 12h or 24h format was
configured.

Bug: 32761619
Test: m RunSettingsRoboTests
Change-Id: Idbbb8f79fccec95e33bf2f12767d5736e1118fa7