Use date difference to remove the expired data

Previously we use absolute milliseconds difference to remove the data
and it's not aligned with the assumption we made. Update to use the date
difference when removing the expired data.

Also, add more test cases for getUserCategory() method.

Test: atest HearingAidStatsLogUtilsTest
Bug: 325699522
Change-Id: I20fd2774c732cdc7fa16f81ff554e30a146d08f9