Modify j.t.DateFormat.set24HourTimePref() to take a Boolean

There is a legitimate state "unset" which means
"use the Locale default". The method took a boolean primitive
which meant it was not possible to reflect this state.

This change is required to support modifications in
ActivityThread to correctly handle the "unset" state.

Bug: 32868036
Test: Manual testing of app behavior after a factory reset
Change-Id: Iee2c15390e11e3f9073d5c00bb94c3879bb9834e