Prework for migrating min_sdk_version from (kind+level) to (level)

This relands aosp/2465355. The original CL was submitted as part of a
stack which broke tm and udc. Those branches contain soong modules with
min_sdk_version of type (kind+level)

Bug: 208456999
Test: go test ./sdk (top of CL stack)
Change-Id: I627f69b1fd71ab386e9026a2fc1566bad786bf1d