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

java/java.go

@@ -2526,6 +2526,10 @@ func (j *Import) sdkVersion() sdkSpec {
 	return sdkSpecFrom(String(j.properties.Sdk_version))
 }
 
+func (j *Import) makeSdkVersion() string {
+	return j.sdkVersion().raw
+}
+
 func (j *Import) minSdkVersion() sdkSpec {
 	return j.sdkVersion()
 }