Fix robotest failure for ChartDataUsagePreferenceTest.

- the chart usage data is computed using difference between current time
and the timestamp from the usage data. In the test, we are using actual
current time and mock usage data. As the current time varies the test
computating sometimes results in some negative data, and makes the
comparison unpredictable. Change to use a fix "current" time in the test
so that the time difference in computation will be constant.

Fixes: 77824818
Test: make RunSettingsRoboTests
Change-Id: I2d8c166839775469d30015317afdc04eb3d9a2b4