Default annotations_enabled to true for sdk_library

This turns out retention of Nullable annotations, which is almost
certainly what every user wants. Also add the "stub-annotations" lib to
the stub libs property, to make sure the nullability annotations
resolve properly.

Bug: 157010342
Test: m
Change-Id: I905144add4ef6f85445eedd1ba5169445f6005e0

sdk/java_sdk_test.go

@@ -61,17 +61,20 @@ java_import {
 	name: "android_module_lib_stubs_current",
 }
 java_import {
-	name: "core-lambda-stubs", 
+	name: "core-lambda-stubs",
 	sdk_version: "none",
 }
 java_import {
-	name: "ext", 
+	name: "ext",
 	sdk_version: "none",
 }
 java_import {
-	name: "framework", 
+	name: "framework",
 	sdk_version: "none",
 }
+java_library {
+	name: "stub-annotations",
+}
 ` + bp
 
 	return testSdkWithFs(t, bp, fs)