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

java/testing.go

@@ -176,6 +176,7 @@ func GatherRequiredDepsForTest() string {
 		"kotlin-stdlib-jdk7",
 		"kotlin-stdlib-jdk8",
 		"kotlin-annotations",
+		"stub-annotations",
 	}
 
 	for _, extra := range extraModules {