Separate the experimental DHCP features setting in IpClientIntegrationTest.

Move the specific implemenation of the experimental flag enable/disable
to IpClientIntegrationTest and IpClienRootTest separately, the benefit
would be to enable more test cases running on RootTests without the
interference of experiment flags.

Bug: 138926174
Test: atest NetworkStackIntegrationTests
Change-Id: I439c0688b17d5d5883ad25148ced11f585e9bce8