Return MobileElement directly from AppiumDriver.findElement?() and findElements?() methods

I found out that we can make ```AppiumDriver``` to convert element Json representation directly to ```MobileElement``` rather than ```RemoteWebElement```. So we don't need to wrap ```RemoteWebElement``` inside ```MobileElement``` but just use ```MobileElement``` as a rightful successor of ```RemoteWebElement```.

Instead of:
```java
MobileElement el1 = new MobileElement((RemoteWebElement)driver.findElementByAndroidUIAutomator("resourceId(\"android:id/content\")"), driver);
```
we can write down it in more simple way as :
```java
MobileElement el1 = (MobileElement) driver.findElementByAndroidUIAutomator("resourceId(\"android:id/content\")");
```
and also we can continue searching to find a child element under ```el1``` (means use any methods available inside ```MobileElement``` class):
```java
MobileElement el2 = (MobileElement) el1.findElementByAndroidUIAutomator("text(\"Accessibility\")");
```