Fix issue where line alpha would be overridden

A Paint doesn't have separate components for color and alpha, so when
setting a color, the alpha will be lost.

We need to take that into account when updating the tint.

Test: manual
Test: atest SquigglyProgressTest
Fixes: 227388771
Change-Id: Ibb85305a01c552c1476e502d23614de097f4680c