Allow retrieving info /system components

Generally we return info for the latest installed package; which could
either be a built-in [i.e. on the /system partition] package or a user
updated package. In certain circumstances, we want to be able to get
the version on the /system partition regardless of whether or not the
user has updated it. We do this by passing MATCH_FACTORY_ONLY to
getPackageInfo().

Bug: 27469181
Change-Id: I8dd1d110e2d72e5c6f024812d0b5d15d8b217347