AutoProtoSchemaBuilder.schemaFilePath() is not handled correctly

Current implementation is wrong in these aspects:

• the (optional) schemaFilePath together with schemaFileName should only be used for constructing the classloader resource path
• when computing the final resource path from schemaFilePath + schemaFileName should take into account eventual leading or trailing '/' and handle it gracefully
• the file path used for SerializationContext registration should be only based on schemaFileName, and should not be relative to schemaFilePath