Reduce visibility on PackageDexUsage methods.

Not a big win, but it makes it clearer that the data is never directly
changed by PackageManager etc.  (We could extract the two nested
classes and then PackageDexUsage wouldn't need to be public at all.)

Test: Still builds.
Test: atest services/tests/servicestests/src/com/android/server/pm/dex

Change-Id: I0a5c0bff473a03178f730e354a2f60694729ba10