Refactor processBatteryDiffData() from DataProcessor to BatteryDiffData class.

So that only hourly battery diff data needs purgeFakeAndHiddenPackages()
and combineBatteryDiffEntry(). This is also easy to set hourly threshold
in the next cl.

Bug: 264840285
Test: manual
Change-Id: Ie0bc6d53f96285285019dd83c1f39305eca79c71