Adding possibility to send userProperties to UIAutomator

Today we're missing a way to send custom properties to the UIAutomator tests.

My suggestion is to send custom properties by specifying a prefix for standard maven properties with the node "propertiesKeyPrefix".

Example:
```
<uiautomator>
	...
	<propertiesKeyPrefix>Prefix</propertiesKeyPrefix>
</uiautomator>
```

```
mvn <goal> -DPrefixKey=Value
```

Then it will be possible to fetch the custom user properties with
```
getParams().getString("Key")
```
within the UiAutomatorTestCase.