Tint the pointer white at all times.

Currently Bubble backgrounds are also white at all times (we think for background protection reasons?), so it makes sense to align these. If we reopen b/153367093 with a fix that allows for dark-backgrounded bubbles in dark mode, I'll apply the same fix here.

Test: manual
Fixes: 130834403
Change-Id: I92a81b8c224cbb2ac580bdfc4e40bdd2744d353e