UserSystemPackageInstaller intern packageNames

UserSystemPackageInstaller obtains a list of package names
from a sysconfig file (the whitelist) at boot time.
But package names are already interned in PackageParser.
So we should intern the package names here too to save memory.

Bug: 134605778
Test: atest UserSystemPackageInstallerTest
Change-Id: Icf519473122f6b758749286b5efa8989c6b5ee60