Add proper validation to relevant setter methods of upload task files.

Files to add validations:

1. LambdaTestTask.java
2. LambdaUploaderTask.java

For username and key validations, check for null/empty values and raise appropriate errors. For appFilePath and testSuiteFilePath check for the possible scenarios and update accordingly. As per docs we can also pass app and test suite IDs instead of paths.