Update min_sdk_version used in TestSymlinksFromApexToSystem

Updatable apexes should set a min_sdk_version that is not current. This
enforcement does not exist today, but will soon be added to the build
system. In preparation for that, cleanup the min_sdk_version property
used in this unit test.

Test: go test ./apex
Bug: 221087384

Change-Id: Idabc4c6f20e134b38ed0ae3ce68f6b7d54d7a13c