Delete buildIaPdOption and introduce PrefixDelegation#build

There is no reason to have a static helper function. Moving this to
PrefixDelegation is better OO. Encapsulate both decode and build methods
for IA_PD options inside PrefixDelegation class.

Test: atest NetworkStackIntegrationTests
Change-Id: I6ffdbb3701449c54ef7db0302fb0a32710ceb722
3 files changed