Check in built python library.

Add built python library using the following steps:
1. Download libprotobuf from
https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protobuf-all-3.6.1.zip.
2. Goto protobuf-all-3.6.1/python
3. python setup.py build --build-base=[output_folder].
4. Copy output_folder to prebuilts/libprotobuf/linux/python
5. Copy setup.py from protobuf-all-3.6.1/python to
prebuilts/libprotobuf/linux/python
6. Copy src folder from protobuf-all-3.6.1 to
prebuilts/libprotobuf/linux/python, this is necessary for unit
tests.

Change-Id: Ib51c7b43051b809c784fcca93f9d0290bcdaecb4
Tests: Unit tests run using 'python setup.py test' under python
folder.
Bug: b/80498777