Migrate first IpClientIntegrationTests to root

Instead of using the network_stack certificate, tests in
IpClientIntegrationTest should use root access. This allows running them
on any userdebug device, while the network_stack certificate requirement
is more restrictive.

This change renames IpClientIntegrationTest to
IpClientIntegrationTestBase, which IpClientIntegrationTest and
IpClientRootTest inherit from.
Current tests in IpClientIntegrationTestBase are run in IpClientRootTest
by default (root access should be available when signature permissions
are available), and only run in IpClientIntegrationTest when annotated
with @SignatureRequiredTest.

This change adds @SignatureRequiredTest to all tests except
testDhcpInit. This causes current tests to run in the same configuration
as today. testDhcpInit demonstrates the migration by running in
IpClientRootTest.

Bug: 160541918
Test: atest NetworkStackIntegrationTests
Change-Id: Id4e714176c923d4cbe0dc100d2d472362644a509