HeadsetTest: Use injected test looper and Truth

By using injected test looper, we can now remove the need to use sleep
in test that induced flakiness
By moving to truth we can now have better error message when an
exception is thrown

Fix flakiness >5K iteration on each passed, before it failed in the
first 100

Remove check "isOnStateMachineThread" as this is silently discarding
message and not compatible with a TestLooper created in Setup

Test: atest HeadsetServiceAndStateMachineTest --rerun-until-failure
Test: atest HeadsetServiceTest --rerun-until-failure
Test: atest HeadsetStateMachineTest --rerun-until-failure
Flag: Exempt, only modify logic in test
Bug: 338303046
Change-Id: I0507fe4bb9e4a3abe4bb1be1defa7e2435566635