Move letterboxing to AppWinToken

Both letterboxes for filling the DisplayCutout area and to satisfy
maxAspectRatio constraints are moved to AppWindowToken.

Bug: 65689439
Test: adb shell cmd overlay enable com.android.internal.display.cutout.emulation && adb shell stop && adb shell start; verify Launcher has a black bar
Test: on a 18:9 device, open a legacy app. Ensure it has a proper max aspect ratio letterbox.
Change-Id: I8fd7802ce5ce14b8aa71232c62dc4283351b3021