Move conv_classpaths.proto to packages/modules/common.

The classpaths.proto file is needed at build time to generate appropriate
classpath configs for the modules. Unbundled ART branches do not include
packages/modules/SdkExtenstion in their manifest; so put the proto in a
project that should be visible to all unbundled modules.

Bug: 180105615
Test: m nothing
Change-Id: Ie5be1822fbee92f280332622a9317894cc98d7ac