Split app name and version in Capabilities.

To enhance capability matching and to select the latest version of the app to test if the
app version is not provided by the users. This enhancement still retains the backward 
compatibility when users specified an AUT as "appName:appVersion".

Signed-off-by: Binh Vu <bvu@paypal.com>
This function just finds the latest version of the app in the apps store when the app version is not specified in the requests. SelendroidStandaloneDriver already checks and raises SessionNotCreatedException if the request application is not in the apps store.