xds: ensure server interceptors are created in a sync context

[sergiitk]

XdsServerWrapper#generatePerRouteInterceptors was always intended to be executed within a sync context. This PR ensures that by calling syncContext.throwIfNotInThisSynchronizationContext().

This change is needed for upcoming xDS filter state retention (PR #11936) because the new tests in XdsServerWrapperTest flake with this NPE:

Cannot invoke "io.grpc.xds.client.XdsClient$ResourceWatcher.onChanged(io.grpc.xds.client.XdsClient$ResourceUpdate)" because "this.ldsWatcher" is null

[sergiitk]

Sidenote. I've provided FakeXdsClient#awaitRds and FakeXdsClient#setExpectedRdsCount purely for the reason of not having to explain how to await for RDS in the comment to execute.

But when I was replacing xdsClient.rdsCount.await(5, TimeUnit.SECONDS); with xdsClient.awaitRds() in all tests, I found the reason the tests got stuck running forever the other day:

grpc-java/xds/src/test/java/io/grpc/xds/XdsServerWrapperTest.java

Line 691 in cdab410

xdsClient.rdsCount.await();

Oh, Java, and its timeout-less waits.

[sergiitk]

For posterity. Windows job had XdsClientFallbackTest failing, probably not related.

Type with non-zero subscribers is: `type.googleapis.com/envoy.config.cluster.v3.Cluster`
expected: 0
but was : 1
	at app//io.grpc.xds.XdsClientFallbackTest.verifyNoSubscribers(XdsClientFallbackTest.java:337)
	at app//io.grpc.xds.XdsClientFallbackTest.mainDown_fallbackUp_restart_main(XdsClientFallbackTest.java:322)

https://source.cloud.google.com/results/invocations/237fded9-83ba-4041-8c76-0b14bdf3787d