Provide 0 insets to VoiceInteractionWindow in gesture nav

Test: Invoke assistant on tablet on both geture nav and 3 button mode;
In gesture nav mode: assistant shows at the very bottom of the screen,
layered on top of the taskbar background (in z order)
In 3 button mode: assistant shows right above the taskbar, which only
shows the 3 buttons (no taskbar icons) while assistant is showing
Fixes: 225200928

Change-Id: I2fbc05e696b58e8801e704092aa2741cb57c05c4