Change message_registries namespace to registries

The message_registries namespace is overly wordy, and results in very
long defines.  Doing this one minor change reduces the code by 50 lines.
This seems worthwhile.

Tested: Unit tests pass.  Namespace change only.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ib1401580b3fa47596eb56cdc86e60eeeb1c2f952