Add running apps icons to taskbar for desktop environment.

This CL adds app icons for launched/running apps to the Launcher
taskbar hotseat. When the activity is closed, the app icon is
removed. The apps that are added to the taskbar on boot are never
removed.

Recall: http://recall/clips/ad6d3cfc-7358-4b37-846e-de843ad3000d

Bug: 183906774
Test: Launch an app and verify the app icon is added on the taskbar.
Close the app and verify the icon is removed from the taskbar.
Test: Switch navigation modes on the emulator and ensure that running
app icons are added to the taskbar after it is reinitialized.
Test: atest NexusLauncherTests:com.android.quickstep.RecentTasksListTest RecentTasksControllerTest

Change-Id: I2c839b6f35310d2d0fa98cc8d4a432eb1163861d
Merged-In: I2c839b6f35310d2d0fa98cc8d4a432eb1163861d