BasicNativeAndroidTest: Move gtest to a single flavor

[utzcoz]

The junit-gtest library will bring native test library and its c++ shared library into the final package. It's not easy to keep these necessary shared libraries into androidTest flavor only. So this CL adds a new flavor called nativeTest to include native test related libraries and run it with androidTest. For normal usage, we can use core flavor, the new default flavor without any native test related libraries.

[utzcoz]

cc @fsladkey and @brettchabot for reviewing and discussion. I am experimenting junit-gtest, and I want to find a better approach to resolve junit-gtest packaged so files, and if Google has more experience, really hope to listen your response.