IpClient: Only use DTIM grace period when SLAAC is used

In IPv6 link-local only mode, the system does not accept RAs, so there
is no reason to lower the DTIM multiplier for IPv6 provisioning.
The same goes for when IPv6 is disabled. And update the integration
tests.

Bug: 266256943
Bug: 280544667
Test: atest NetworkStackIntegrationTests
Change-Id: I4e815981be7f1c6f4581649625162debb603d3d0
2 files changed