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

Currently, the following lib will be included in the sdk snapshot even
though it is compiling against private apis
```
java_sdk_library {
  name: "foo",
  //sdk_version defaults to "",  i.e. SdkSpecPrivate
}
```

This is because min_sdk_version of `foo` inherits the api level of
SdkSpecPrivate (i.e. FutureApiLevel).

As part of the migration of min_sdk_version to ApiLevel, the api level
of SdkSpecPrivate will be different than FutureApiLevel. In the above
example, assuming the version of the sdk_snapshot is FutureApiLevel,
`foo` will be included only if it explicitly sets a min_sdk_version <=
FutureApiLevel. Update an existing unit test to set this value
explicitly.

Bug: 208456999
Test: go test ./sdk (top of CL stack)

Change-Id: Iddd497df7da8c829325d902fbf70731dd8c6855d

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 {