Check and Add the `@Singleton` Annotation to All Stores

[ParaskP7]

As part of this PR and this change, a discussion was started as to whether the PluginCoroutineStore that was missing the @Singleton annotation should have it, just like the rest of the stores (as per the CheckStyle rule here).

Also, from that discussion, two more stores were identified with the same problem, the SiteOptionsStore and XPostsStore stores. The @Singleton annotation is missing there as well.

This issue is about checking whether the@Signleton annotation that is missing on those 3 stores above (and potentially others) was deliberately done so, or else accidentally missed. Then, the @Signleton annotation should be added to this stores and tested for correctness (via the example and client apps, WPAndroid and/or WCAndroid).