DisplayCutout: Properly letterbox bottom-only cutouts

Fixes an issue where bottom cutouts where not
letterboxed if there is no need to letterbox the top

Instead, the need to letterbox is derived from the
bounds.

Change-Id: I6053053066473bd78f21df978e6704043a85e80b
Fixes: 75024814
Test: Receive incoming call in double cutout mode, verify bottom of dialer is letterboxed.