I have set my keyboard to have a done key and that seems to replace the Return key. Is it possible to have both? Or if you have return, does the user need to use the devices back key to exit the keyboard?
As LucaMS suggested you could add a button to the layout to close the keyboard, unless you can define a condition upon which it should close, number of lines added or similar, then use the IME library to close the keyboard from the button or condition.
You could try managing the visibility of the close button with the FocusChanged method on the edittext.
On my Galaxy S3, when using multiline, I just press the back key to exit the keyboard when I'm finished typing. I'd assume that all Android phones worked the same.