Letterbox: configurable rounded corners.

Also, allow a transparent appearance of status and navigation bars when rounded corners are enabled for test purposes. The final goal is to make status bar a semi-transparent black in the letterbox mode when a transparent status bar is requested even when corners aren't rounded.

To enable rounded corners run (e.g. set radius to 50):
adb shell cmd window set-letterbox-activity-corners-radius <radius>

To get corners radius run:
adb shell cmd window get-letterbox-activity-corners-radius

To reset corners radius run:
adb shell cmd window set-letterbox-activity-corners-radius reset

If radius <= 0 then rounded corners will be disabled.

Test: adb shell cmd window (set|get)-letterbox-activity-corners-radius
Bug: 170216100
Change-Id: I0cbd3d0966f80d3623451a40b7f073496d2cc516