Bind voice interactor as top-app when it is being used

When voice interactor is being used, it should run as the top app to
handle processing as quickly as possible.  This is similar to what IME
does when IME is shown.

Bug 139756725
Test: invoke assistant, confirm :interactor becomes top-app and then
goes back to foreground after use.

Change-Id: Id13b47dcb4de6658eede13a650e6ef631f649f4f