[tflite] delegate cos to NNAPI
@gbaned and @qukhan could you tell me why Google internal checks FAILED? The failures are unrelated to your pull request. I'm working on getting around the issue.
We need cosine for some networks, such as transformer positional encoding. Cosine is not directly supported by NNAPI, but we know $cos(x) = sin(\frac{\pi}{2} - x)$