Fix the contrast ratio between the icon and screensaver background

The contrast ratio is below 3:1 when the screensaver has a nearly
white background. Tweaked the drop shadows to make the contrast
meet accessibility requirements.

Fix: 380229099
Flag: NONE bug fix
Test: manually with WebAIM
Change-Id: I2ee4b49c0ddfe8fcfe8f9c38e7e6e66efb300318