Add rounded corners to letterbox when taskbar is visible

Previously letterbox corners were not rounded
when taskbar was minimized.

This is because taskbar insets height was
smaller than taskbar_frame_height

This check is not necessary, and crop
of the application window is applied correctly

Change-Id: Ib3f0737b6cfd593a7bf3dbf9205e4cc9a3f9b506
Bug: 259679233
Test: manual