AOD: Increase alpha values when dozing

Fixes an issue where notification icons would show weirdly because
the low-power mode for AOD shows <50% white as black. As a workaround
we boost alpha such that >30% white shows as 50% white and thus is
still visible as full white on AOD.

Change-Id: Ida873b97be191e5d75d6029054ba5b931726ce4f
Fixes: 63742595
Test: Disconnect from wifi to trigger the "wifi networks available" notification. Make sure its icon shows properly on AOD1.