Override assistantOnTopOfDream config for tv

Assistant  should show on top of the Dream on TVs. On phones the Dream
is on top. This CL overrides the default value of the
assistantOnTopOfDream config.

Bug: 152425699

Test: m && flashall && verify that assistant shows on top of dream on tv

Change-Id: If48a8f599bb4ddee185dec0aaaba94f07ccec429