Make aspect ratio of task letterboxing configurable.

Fix: 170215894
Test: go/wm-smoke
Test: manual with 'adb shell cmd window set-task-letterbox-aspect-ratio'
and 'adb shell cmd window set-task-letterbox-aspect-ratio'

Change-Id: I4ea35f065dd20dd2bfbf813734beed633cb3a101
5 files changed