PackageManagerService: Change getCompressedFiles to reflect the new layout.

Stubs are compressed artifacts are installed in sibling directories.

The layout of a compressed package on a given partition is as follows :

Compressed artifacts:

/partition/ModuleName/foo.gz
/partation/ModuleName/bar.gz

// Stub artifact:

/partition/ModuleName-Stub/ModuleName-Stub.apk

In other words, stub is on the same partition as the compressed artifacts
and in a directory that's suffixed with "-Stub".

Bug: 63802184
Test: Manual

Change-Id: If7154c985956ae982ce40c8ede763b1c8e0175a9