Add a config parameter for the max num of tasks

The maximum number of active tasks on a given Desktop Windowing session
should be a device-dependent value. This creates a device-dependent
config for this, and stops limiting the number of tasks if the value is
0 or less.

Fixes: 354156740
Fixes: 335131008
Test: atest DesktopTasksLimiterTest
Flag: com.android.window.flags.enable_desktop_windowing_task_limit
Change-Id: I3ab6c1f9c78ed834da978803aac3b63388d99530