Drop persisted ledgers for uninstalled packages.

Make sure we don't continue to include data from missing apps when
loading data from disk. This scenario could arise if an app is
uninstalled and the device shuts down before we have a chance to
persist the change.

Bug: 158300259
Test: atest FrameworksMockingServicesTests:ScribeTest
Change-Id: Ibebd5296a5e1968ceeb916fdc076fad994c48373