[Desktop Mode] Show indicators under Taskbar app icons for running apps

In Desktop Mode we show running Desktop apps in the Taskbar. With this
change we also show an indicator under the app icon for each such
running app.

Bug: 332504528
Test: manual
Test: DesktopTaskbarRunningAppsControllerTest
Flag: ACONFIG com.android.window.flags.Flags.enableDesktopWindowingTaskbarRunningApps DEVELOPMENT
Change-Id: If0906dab8ad0bd8a78d93a4e99db47550e763bed