Improve documentation for IntentService

The argument to onHandleIntent(Intent) is sometimes null. This
is surprising, and isn't mentioned in the Javadoc.

+ Add a warning about this behavior to the method's Javadoc.

+ Add @Nullable annotation so tools that do null safety analysis
  (e.g. Android Studio) will generate a warning for apps that
  dereference the intent parameter without checking.

Change-Id: I26485eba8c79ba700dd9423e5d356efbb423becd