Taking userId into account for fixupAppDir().

We previously used the getPackage(packageName) API, which doesn't take
the userId into account; this could result in passing in the wrong uid
into fixupAppDir(), because fixupAppDir() validates the passed in uid
matches with the path.

Instead, use PackageManager.getPackageUidAsUser() to match the userId
that was passed in through the path.

Note that we don't care about paths which don't have a userId embedded,
since those are public volumes, for which we don't need this API anyway
(since public volumes don't have UID/GID etc.)

Bug: 157271022
Test: pm create-user aap ; log in to user, try selecting ringtones
Change-Id: Id8731cb18c16c4742d232048f9093fbd57323159