rxjava-android Poposal: Call action immediately in HandlerThreadScheduler if thread is the same

By avoiding unnecessary Runnable creation, and posting it to handler we can decrease overhead when scheduling is made from the same thread.

Current Activity.runOnUIThread is implemented in the same way:
```java
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }
```