[inc] Test vendor API level instead of system for IncFS version

IncFS is a kernel feature, and kernel is a part of a vendor
image. That's why test must account for vendor freeze and only
check the vendor API level

Bug: 183579236
Test: atest PackageManagerShellCommandIncrementalTest

Change-Id: I5b188a090c10ea1a4104c9a3466297349edc48c4
1 file changed