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.