Force PMS snapshots

Removes the ability to disable PackageManagerService snapshots. The
new PackageState and related APIs rely on never taking PMS#mLock, so
a snapshot Computer must always be available.

To keep changes small, this does not migrate existing
executeWithConsistentComputer calls, but that can be done in a simple
follow-up to save on lambda allocation costs.

Bug: 202291547

Change-Id: I243988b0c48938a8701842a9de5cfc465c1d38d8