Add workaround for empty system routes in new proxy routers

This CL adds a hack to work around b/339583417, which introduces
unrelated flakiness in the tests.

Bug: b/339583417
Bug: b/362507305
Test: atest CtsMediaRouterTestCases:android.media.router.cts.SystemMediaRoutingTest
Flag: EXEMPT test only change
Change-Id: Icd08b38a0d5d5223c80fa570ec910f8265217332