Allow status and navigation bars to be semi-transparent in letterbox mode.

Before this change, opaque bars were drawn over letterboxed activities to prevent showing a transparent bar over them because that can make notification icons or navigation buttons unreadable due to contrast between letterbox background and an activity. For instance, this happens when letterbox background is solid black while activity is white. To imporve UI in this case, semi-transparent bars are now allowed to be drawn over letterboxed activities.

Test: atest SystemUITests + manual letterbox testing
Fix: 175482966
Fix: 170216100
Change-Id: I35e35d01df7d25a5d77eb64af4e6b800f5020487