Demo app for multi-window multi-device mode

Add an app that has two windows where you can draw. It showcases the new
feature of drawing into two windows with two different devices (touch
and stylus) at the same time.

Previously, this feature was not available on the platform.

Bug: 211379801
Test: APP=MultiDeviceInput; m $APP && adb install $ANDROID_PRODUCT_OUT/system/app/$APP/$APP.apk
Change-Id: I68875e3bba08a9e5ad5c917866f132e91b7c032e