Make testMultipleMessagesDispatched slightly less overspecified.

LooperStatsTest#testMultipleMessagesDispatched seems to fail all
the time in presubmit due to the test expecting
ExternalSyntheticLambda5 instead of ExternalSyntheticLambda4.

The test will still fail if lambda naming conventions change, but
at least it will be slightly more robust.

Test: treehugger
Change-Id: Ibdd866a29ab7335c7f9149702de3735c94036111