Skip Route tests that are R-specific on Q

MTU was added in R so test for it should be run on Q. Also
revise test to adopt rule.

Test: atest CtsNetTestCasesLatestSdk:android.net.LinkAddressTest
      on both devices
Bug: 150918852
Change-Id: Ibab9cfae0d35b26c6e4ca0defbb89769b04201d4