[Thread] Mark ThreadConfiguration#isDhcpv6PdEnabled() as @hide

This CL marks ThreadConfiguration#isDhcpv6PdEnabled() as @hide because
there's no way yet to set it as enabled. When PD is implemented, we'll
expose this API as System API.

Bug: 342519412

Change-Id: I6b8f7b83bd01c0ba045c777e6f613bc4a74d9dfb
2 files changed