Dialer: fix wrong visible status of voice search button

When user enters dialer main screen, it will prepare and check
whether should show voice search button, but after back to non-search
mode from search mode, it ignores to check and so the button status
become wrong.

We add to check whether should show voice search button in visibility
updating function.

Change-Id: I4374b3c44156294d00d5290316a7510e41a331a1
CRs-Fixed: 730644