[DataUsage] Adjusting the control of display sequence

Showing a usage graph before end of statistic would lead to incorrect
height of usage graph, and another update would lead to layout moved a
little bit.

This change tries to improve it through:
1. Start the loading animation earlier, and stop animation when
   statistics loaded.
   (Only effective when UI re-create.)
2. Update the UI only when statistics are ready.

Bug: 187019210
Test: robotest ChartDataUsagePreferenceTest DataUsageListTest
Change-Id: Ic83f2422b6c6d55948110d652ee24234f43b6445