Adding addtional parameters for startApp and removing handling of variab...

-Adding new android capabilities for [Issue #2026] (https://github.com/appium/appium/issues/2026)
-Now users will have flexibility to provide action (default `android.intent.action.MAIN`),category (default ``android.intent.action.MAIN``) and flags (default `0x10200000`).
-Also added 'optionalIntentArguments' capability.
-Removed lines which are use to handle optional parameters, will need to ensure all parameters are passed from appium, this will help easy extension of this function if needed.  
in the case where optionalIntentArguments is empty, the cmd var will have a trailing space. Probably best to ensure it doesn't somehow.