Taskbar running apps: show one icon per task + tap opens that task

Before this CL we would show only one Taskbar icon per app, making it
impossible (through the taskbar) to switch between several windows of
the same app.
With this CL we add one icon per task instead, making it possible to
bring each task to the front individually.

Flag: com.android.window.flags.enable_desktop_windowing_taskbar_running_apps
Bug: 351118893
Bug: 349790805
Bug: 351156858
Test: Started several Chrome windows -> taskbar has one icon per window

Change-Id: Ia692977effceb9ce339906bf6ca24d73e19d8769