[A11Y] Improve data usage chart TalkBack content

1. Set null content description for all labels of the chart.
2. Set 2 parts in the content description of the chart:
  I.  Brief description of the chart.
  II. Stats of the data usage.

Bug: 141093026
Test: ChartDataUsagePreferenceTest
      Manually listen to TalkBack speaking
Change-Id: I82cefd9987793f40a5bba5bf3ea5f4017da37640