Avoid unnecessary Clock GC churn

Avoid recreating SimpleDateFormat and DateTimePatternGenerator objects
for each clock update. While the allocation sizes are relatively small,
they are continuous, and potentially frequent when seconds are shown.

Test: atest SystemUITests:StatusBarTest + visual inspection
Bug: 204187068
Change-Id: Ia267c6df44002328c6c32115326dc092ee89393f
1 file changed