Make hideKeyboard do nothing when keyboard is present but not closable

`hideKeyboard` performs a back-button press to hide the keyboard UI, but it will do so even when the keyboard has no UI to back out of (hardware keyboard, Appium UnicodeIME). This pull request relies on appium/appium-adb#59 to provide additional information about whether or not the current keyboard has a closeable UI. If there is no UI to close, `hideKeyboard` simply returns successfully.