Generate stack override config in AM vs. WM

First step in allowing activity manager to drive the override
configuration of stacks at creation time vs. using what window manager
returns. Also moves us in the direction where callers outside of AM/WM
will eventually be able to drive the window configuration.

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: I50bc9e0634779142c173e501d16e7aa1b71f28ce