Fix issue related to Average Label in Landscape Graphs

[Ramshandilya]

Fix issue where the average label doesn't change with time scale changes in Landscape graph.

• Pull corresponding unit from the HKUnit property in APCScoring.
• Add support to hide/unhide the units.
• Add helper methods for average/Min/Max value strings.

[Ramshandilya]

Done :). Thanks @chrisinsfo.

[ghost]

@YuanZhu-apple We believe this is good to go.

[YuanZhu-apple]

r＝me.