Introducing windowing mode in WindowConfiguration.

Currently Stacks (specifically their ids) are used to determine
windowing mode for activities. This isn't flexible when it comes
to changing/mixing various windowing modes at different levels
of the window hierarchy. E.g. how do you have the non-default
display support freeform or split-screen or how do you
interleave freeform windows with fullscreen windows.
To help with this problem we are introducing windowing mode
in WindowConfiguration that can be used to set the windowing
mode for any WindowContainer in the hierarchy.

Currently all displays are set to fullscreen windowing mode and
stacks windowing modes are set based on their id.

Test: bit FrameworksServicesTests:com.android.server.wm.WindowConfigurationTests
Test: adb shell am instrument -w -e package com.android.server.wm com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Test: go/wm-smoke
Change-Id: Iccdc3212cda651998d6ad76ce5261d089bff897a