Hide ipv6 subnet API attributes

The attributes for ra mode and address mode should be hidden
until the IPv6 feature is fully implemented.

The changes in this patch will be reverted by another patch
which closes this bug.

Change-Id: I69a1a571b5beb566641200e60b84f0716c1ec138
Related-Bug: 1304093
(cherry picked from commit d739486a3f70cb83f8730cf7201a8b8ccf4b4c70)
Merge "Hide ipv6 subnet API attributes" into milestone-proposed