Add ota_metadata_pb2 to otatools.zip

Sometimes tests are being run by executing python -m unittest *.py. In
that case, invocation will fail due to missing ota_metadata_pb2, which
is auto generated from ota_metadata.proto(added in recent change).

Bug: 166718341
Test: unzip otatools.zip and python releasetools/test_utils.py
Change-Id: I597c7ef6dbd8e46ccde020b3122cc38a8df28dc9