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

sdk/bootclasspath_fragment_sdk_test.go

@@ -839,6 +839,7 @@ func TestSnapshotWithBootclasspathFragment_HiddenAPI(t *testing.T) {
 				compile_dex: true,
 				public: {enabled: true},
 				permitted_packages: ["mysdklibrary"],
+				min_sdk_version: "current",
 			}
 
 			java_sdk_library {