Fix flaky TestWakeupClientServiceImplUnitTest.

This CL fixed two possible issues: 1. mServerStarted is not reset
to false for each test case. 2. pollOnce should be used instead of
pollAll since pollAll might swallow wake.

Test: atest --host TestWakeupClientServerHostUnitTest
Bug: 333673876
Change-Id: Id705d6257c54a2e2711aff6018abf2ea65b0f691