Add imports_sdk_version to prebuilt_apis

This allows setting the prebuilt sdk's generated java_imports
sdk_version to "none".

Test: go tests
Test: m
Bug: 160455085
Change-Id: Id24aca811f0f09692971e63418da6685d8351737
diff --git a/java/androidmk.go b/java/androidmk.go
index bc327cf..5cb0426 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -196,7 +196,7 @@
 				entries.SetBool("LOCAL_UNINSTALLABLE_MODULE", !Bool(prebuilt.properties.Installable))
 				entries.SetPath("LOCAL_SOONG_HEADER_JAR", prebuilt.combinedClasspathFile)
 				entries.SetPath("LOCAL_SOONG_CLASSES_JAR", prebuilt.combinedClasspathFile)
-				entries.SetString("LOCAL_SDK_VERSION", prebuilt.sdkVersion().raw)
+				entries.SetString("LOCAL_SDK_VERSION", prebuilt.makeSdkVersion())
 				entries.SetString("LOCAL_MODULE_STEM", prebuilt.Stem())
 			},
 		},