Just get VoiceInteractionManagerService once in Activity.java

During testing the testActivityTransitionsAnimation, it usually
takes 400us ~ 1ms to get the VoiceInteractionManagerService, it
would be better to get VoiceInteractionManagerService once
and keep it instead of getting VoiceInteractionManagerService
again in the other stages.

Test: atest CtsVoiceInteractionTestCases
Bug: 261791888
Change-Id: I46daa11f2281a8db96056ca99520a0c3fe41e4ec