Fix validation URL flakes with DeviceConfigRule

DeviceConfigRule can be used to set DeviceConfig, and retry tests when
flag syncs with server have interfered with the test. This is necessary
before S as there is no mechanism to prevent the device from syncing
flags.

Test: atest CtsNetTestCases
Bug: 210377950
Change-Id: I818cbab76cd4df5da27d4a61d90603412cf80b0b
4 files changed