feat - Android test variant support (#194)

Changes:
- Updated AndroidUtils to populate all fields individually so failure in retrieval of one property won't lead to the collapse of whole source set.
- Added android resource directories only if the variant is not a unit test
- Updated GradleApiConnectorTest#testAndroidSourceSets to consider the all the test variants and their dependencies
- AndroidUtils#convertVariantToGradleSourceSet now returns a list of source sets rather than a single source set.
- AndroidUtils#addTests now returns a list of test source sets.