Remove UsageStats files older than 2 years.

As a privacy improvement, do not store usage stats data that is older
than 2 years. Previously, stats were kept for 3 years.

This will also help reduce the storage taken up by stats and improve the
performance of query stats API calls which require files to be read from
disk since the number of files read are significantly less.

Bug: 148524623
Test: atest UsageStatsDatabaseTest
Change-Id: I159914a87ba4137f0f3d869c69abf3b16ae77657