Enable blur letterbox wallpaper behind flags

Introduces build time and runtime flags to set wallpaper as the default
letterbox background style updating default values for blur radius
and dark scrim alpha.

Updates default color in case of solid letterbox background style.

Bug: 289902667
Test: Manual

Change-Id: Ib6e9388bb1bc5c46ad7ffc545d7f20c8e319fa3c