Fixed uninstallHelperPackage() for headless system user mode.

It was failing because PackageInstaller.uninstall() must be called by
either a DO or an affiliated PO. On headless system user mode, it's
called by a DO, which was not affiliated.

Test: manual verification
Bug: 177554984
Change-Id: Id3a7f4f797aa2783d6c191cab8289bca5f8c49fd
4 files changed