Downgrade protobuf to 2.5 to support IDE integration

we were on 2.6.1 but the IDE including the kotlin plugin
use 2.5 and are not compatible with 2.6.1, as such we
downgrade to 2.5 to be compatible.

Change-Id: Icdefa92736c728c418e9abaae3ed46c8832b8518
245 files changed