Add Dependencies class in Dhcp6Client.

Add static Dependencies class in Dhcp6Client to read the device config
such as the minimal acceptable prefix lifetime, we can inject the
Dependencies class in integration tests and return the expected value
for testing purpose.

Bug: 260934173
Test: atest NetworkStackIntegrationTests
Change-Id: Iaea809eb1fd97de537806c743ff658a582b9d37b
3 files changed