Move protobuf 2.3.0 to prebuilts.

This will facilitate an upgrade to 2.6.0 and future versions without
breaking applications which have a hardcoded dependency on 2.3.0. The
existing make rules have been renamed to omit the version code.

The prebuilts were built from a tree at commit
9be7e7401174d75e87e21fdc5f43549594a714bb.

Change-Id: I90cf7f029217c8dc63a4b38252025c6b725d72a5