PSS: Task Switcher - Client side actions implementation

Switch action: changes the task that is being projected to the current
foreground task.

Go back action: moves to the foreground the task that the user initially
chose to project, but had been moved into the background.

Fixes: 269577804
Fixes: 258176179
Test: Manually
Test: All unit tests under mediaprojection/taskswitcher
Flag: ACONFIG com.android.systemui.pss_task_switcher DEVELOPMENT
Change-Id: Ia8f798e0d316826f4b9432c4f97a90a7a51dcc2f