Add DeepLab

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This pull request adds support for DeepLab on Android. I also modified the example app to compare the different tasks more easily.

[shaqian]

Thank you for the PR. Very cool!
Does DeepLab require the experimental GPU version of tensorflow-lite?

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Thank you for this cool repo! No, I didn't try the GPU version yet. The model is from the GPU example page https://www.tensorflow.org/lite/performance/gpu. It should therefore work with the GPU.

To improve the performance we could also reuse the ByteBuffers https://github.com/shaqian/flutter_tflite/blob/master/android/src/main/java/sq/flutter/tflite/TflitePlugin.java#L255. Just allocate one input and one output when we load the model and reuse them when we call tfLite.run. What do you think?