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
Merge "Hide ipv6 subnet API attributes"