Some cleanups in DataUsageSummary class:

1. Get rid of unused constants.
2. Fix typo in instance variable name.
3. Convert an instance variable to a local variable.
4. Use Java8 lambda for SummaryProviderFactory instance.

Test: make RunSettingsRoboTests -j40
Change-Id: I6cae055d6efd5e724bdd46e04b7077df610a72e9