Refactoring startApp to add callback as a parameter and removing redundant varaibles

-Refactoring startApp to add callback as a parameter and removing redundant varaibles.
-Related to [#2026] (https://github.com/appium/appium/issues/2026)
This seems like a generally useful function. Maybe we should include it up top or in a helper file. If we're just doing it here there's no need to make it a general function; we could just operate directly on `startAppOptions`.