Don't change the voice interactor setting if it's explicitly unset.

Test: manual (currently requires hardcoding the assistant role due to b/175747251)
Test: atest CtsAssistTestCases CtsVoiceInteractionTestCases CtsVoiceSettingsTestCases
Bug: 167261484

Change-Id: Ibc2c1675f8b026f2915c3faf55e8ff3c52e1ee32