Allow unversioned_until: "current".

Since most system libraries aren't actually versioned yet, we
shouldn't be building the stub libraries with version info just yet.
Setting `unversioned_until: "current"` will not use the version
script for anything but android-current (this way when the version
script is applied, we'll immediately catch it if they don't match).

Test: Updated frameworks/wilhelp to use this, make ndk
Bug: https://github.com/android-ndk/ndk/issues/278
Change-Id: I0dab7fd21bb0f063c39368d37feaa3f822e908ce

cc/ndk_library.go

@@ -162,6 +162,14 @@ func shouldUseVersionScript(stub *stubDecorator) (bool, error) {
 		return true, nil
 	}
 
+	if stub.properties.Unversioned_until == "current" {
+		if stub.properties.ApiLevel == "current" {
+			return true, nil
+		} else {
+			return false, nil
+		}
+	}
+
 	if stub.properties.ApiLevel == "current" {
 		return true, nil
 	}