[Settings] Align data usage calculation

Align the calculation of data usage between
DataUsageSummaryPreferenceController and DataUsagePrefereneController.

Bug: 143996139
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=DataUsageSummaryPreferenceControllerTest
Change-Id: I73de79b5cc298bf949827eda0a8bb0274089c016
1 file changed