Allow platform app to get handle to voiceinteraction service.

Address the following denial caused by systemui:
avc:  denied  { find } for service=voiceinteraction pid=10761 uid=10029 scontext=u:r:platform_app:s0:c512,c768 tcontext=u:object_r:voiceinteraction_service:s0 tclass=service_manager permissive=0

Bug: 26842457
Change-Id: I8274d7f31a4390ccfb885389302e4fea9ce0e389