[flexiglass] [bouncer] Fix painting in-progress line on large screens

Correctly adjusts a line end of line being drawn in cases when
screen is large enough for pattern input to be larger than
dot drawing area.

Fix: 414781848
Test: manually verified that pattern is working as expected on a
large screen.
Flag: com.android.systemui.scene_container

Change-Id: Icd50f1268a60fc8231069baa07701e9ea1e2beaf