Set Java version for java_api_library module

Currently, stubs are compiled with Java version 1.8.
To align with this, set the java version passed to javac invocation as
1.8 for java_api_library module. Helper function is used to sync the
java version of droiddoc module and java_api_library module.

Test: m
Change-Id: I103b94aed9747a2f2dd65d13de0ae9c0f33db5c0

java/java.go

@@ -521,6 +521,11 @@ func getJavaVersion(ctx android.ModuleContext, javaVersion string, sdkContext an
 	}
 }
 
+// Java version for stubs generation
+func getStubsJavaVersion() javaVersion {
+	return JAVA_VERSION_8
+}
+
 type javaVersion int
 
 const (
@@ -1718,6 +1723,7 @@ func (al *ApiLibrary) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 	al.stubsJar = android.PathForModuleOut(ctx, ctx.ModuleName(), "android.jar")
 
 	var flags javaBuilderFlags
+	flags.javaVersion = getStubsJavaVersion()
 	flags.javacFlags = strings.Join(al.properties.Javacflags, " ")
 
 	TransformJavaToClasses(ctx, al.stubsJar, 0, android.Paths{},