Add back PackageUsage.java.

This CL adds back the file deleted by
commit 6d255d41.

It's a class that persists the data that backs
PackageStateUnserialized.getLastPackageUsageTimeInMills. At the time of
writing, there is still one use case of the data: instant app cleanup.

Bug: 258223472
Test: Presubmit
Flag: EXEMPT revert
Change-Id: Iadaa3d7dc026916eae699ac438eabec99cbee07c