Detect non-A/B vs. A/B packages correctly.

Check the package metadata to determine whether this is an
A/B or non-A/B update package. This is more accurate.

Also checks ro.virtual_ab.allow_non_ab flag. This is useful for
continuously supporting (and testing) non-A/B.

Bug: 153581609
Test: apply non-A/B update on cuttlefish

Change-Id: I629a533a67966d46d9cd87a59c6b9af26daf1667