Fixed broken isDisplayed() and isEnabled() protocol.

The method [here](https://github.com/appium/appium/blob/623119da6e62b7ecb3f6b4d103efc4c0e7b0a637/app/ios.js#L1351) and [here](https://github.com/appium/appium/blob/623119da6e62b7ecb3f6b4d103efc4c0e7b0a637/app/ios.js#L1362) are returning a boolean value when they should be returning an object with fields status and value.

Prior to this fix, asking an element if it was enabled/visible would always return true as discussed [here](https://github.com/appium/appium/issues/725).