Increase status bar icon shadow alpha values for better contrast

Fixes accessibility issue by increasing shadow alpha values to meet
the 3:1 minimum contrast ratio requirement. Key shadow alpha increased
from 0.35 to 0.8 and ambient shadow alpha from 0.4 to 0.6 for icons
in dream/screensaver mode.

Fixes: 380229099
Test: Device tested
Flag: NONE accessibility fix
Change-Id: If6a720b998d63e221bbf3f4f30380eb958ade6bc