Fix flake in WindowOnBackInvokedDispatcherTest

In rare cases (e.g. when some frames are skipped), it could have happened that no progress update was emitted before onBackCancelled was invoked. This CL ensures that the test doesn't fail in such a case.

Bug: 390528064
Flag: EXEMPT test-only flake fix
Test: WindowOnBackInvokedDispatcherTest
Change-Id: I3c992ef955a4ba11eec1789d812466038d22c1cf