Wait for dispatcher idle before asserting policy callbacks

Before this CL, the policy state was asserted too early. The dispatcher
might not have finished processing the commands yet.

To fix this, force the tests to wait until dispatcher is idle by
providing a dispatcher reference to the assertion function.

Bug: 282837934
Test: atest inputflinger_tests
Change-Id: Ie427071f774d2bfb4b9ebfef94c7971656a69509