Add a way to reset persistent letterbox positions.

It allows to reset currentPositionForHorizontalReachability,
currentPositionForVerticalReachability and the multipliers used when reachability is enabled.

Bug: 285865627
Test: atest WmTests:LetterBoxConfigurationTest | Manual testing with adb wm shell commands.
Change-Id: Ifc98b0c33a01b0593b2801c1ce26eac4f04a2dd8