Use protobuf v2.x

The protobuf crate is being updated to version 3.x. Keep
current users of the protobuf crate on 2.x for now.

This is functionally a no-op.

Test: Treehugger
Bug: 270895633
Change-Id: I92197ea6b91da19b32074b981c5e6f02cb32d82e
1 file changed