Refactor function to support slot size not must be two.

The function insertHourlyUsageDiffDataPerSlot() only supports the slot
has two-hour data now. This assumption is not neccessary. Refactor this
so in my next cl ag/22379150, the last-full-charge slot and now slot
could have only one-hour data.

Bug: 271214926
Test: manual
Change-Id: I57f7898ce1c92b8849baf9a44314455adf0098f4