Remove contents from PackageDataSnapshot

This interface is designed to be an empty type provided to non-PMS
callers to pass into package related APIs. It shouldn't have methods
itself to discourage the access of the data manually outside of those
APIs, since the caller may not be aware of how to cache and use snapshots
appropriately.

Bug: 235834042

Test: atest AppsFilterImplTest

Change-Id: I5e48ef006fdcb242bdcb1e6d0272c5b4e5513e11