Split ST middleware test cases to address flake

The setup method of latency related tests is flaky, which leads to flake
across unit tests, which aren't annotated and don't need the latency
tracker.

Split out the latency tracker related test cases into a separate test
class. Leave those tests marked as flaky.

Bug: 279774505
Test: FrameworkVoiceInteractionTests
Change-Id: Id35624e4ff866056fe986e627a2e55d559049ee4