Trebuchet: Make landscape tablet taskbar start aligned

This fixes the exact same problem that
I293244ffa7037b1e2f9d2c3d5a61f3860bfd45f4 addresses, but in landscape.

The issue was seen on low-resolution tablets, for instance on 1280x800.

Change-Id: I07feae8ad2bcdb35794126a48cdd136a1d13f947
Signed-off-by: Han Sol Jin <hansol@hansol.ca>