Allow unknown codenames in target sdk version for APK-in-apexes

This is currently a recurring issue every new Android release as a few of
our apps will be targeting new platforms and they also get installed on
devices running older SDKs.

This change only changes behaviour for APK in apexes.

Test: atest com.android.server.pm.parsing.PackageParserLegacyCoreTest

Bug: 208239394

Change-Id: If1195c8ec39ac88a605e25dfb1c78d49a5aa3e7c