Fix PerformAdbBackupTask to use getPackageInfoAsUser

When performing an adb backup for a package on a secondary user, the PerformAdbBackupTask was incorrectly using getPackageInfo() without specifying the user.
This caused the PackageManager to look for the package under the wrong user.

This change corrects the behavior by using getPackageInfoAsUser() and providing the correct user ID.
This ensures the package is found and backed up successfully, which resolves the test failure.

Bug: 423803029
Bug: 420527847
Test: atest BackupEligibilityHostSideTest
Flag: EXEMPT bugfix
Change-Id: Ie6666bf7c71ab33103c8dc20ca8fd02232fd8acb