Default keyguard scrim should be 45% transparent

Wallpapers were barely visible on lock screen because value was set to
70%. The unlocked scrim (on the notification shade) was kept at 70% to
improve readability.

Also fixed a bug where scrim opacity wouldn't be overridden via overlay.

Bug: 63868547
Test: visual
Test: atest ScrimControllerTest
Change-Id: I17f277c917dc55604762420c0027b552993f1916