Fix NAN temp on system bar HVAC controls

Fixing bug where the AdjustableTemperatureView displayed "NaN" when no
temperature was available (namely when booting) to instead display "--"
(which was the previous behavior).

Also matched previous temperature view behavior to display a string when
the min or max temperature has been set.

Bug: 168744829
Test: manual - built on flame_car
Test: atest AdjustableTemperatureViewTest
Change-Id: I048cf09fb31b61905c922b7e531f497bee2586d9