Clear invocations earlier in dnd test

This is necessary because dndMode (being a StateFlow), has an extra
"default value" emission at the beginning. Removing
clearInvocations() in the test steps makes it clearer that there
are no unwanted emissions when activating/deactivating DND at any
point afterwards.

Bug: 380908293
Test: ClockEventControllerTest
Flag: TEST_ONLY
Change-Id: I0b630b0bcbba00a8211134eb0ccc070af3af33ca