Fix flaky test for VolumeDialogImplTest.

We only care about whether or not the show/dismiss signals are sent. The
invocation times should be irrelevant.

Test: VolumeDialogImplTest
Bug: 347128147
Flag: com.android.systemui.enable_contextual_tip_for_mute_volume
Change-Id: Ie389b723e3e9ff58de8d90e24f8c93db47d14f7d