Add API to allow apps to update their background color programatically

Allows apps to create a TaskDescription with a custom background color
which they can then set as the app's TaskDescription with a call to
setTaskDescription on the activity.

The background color is used in some window animations and this allows
apps to programatically update this background color.

Bug: 221843305
Test: atest CtsWindowManagerDeviceTestCases:ActivityTransitionTests
Change-Id: If74b7d071a7d57e367080a2e0505254fa6886e90