Add TS flag to control auto task stack controller in WMShell

- The AutoTaskStackController used the flag from Car namespace, which 
  isn't right given the gantry flow for automotive might be different with
  different rules and regulations.
- Instead put the TS flag in wmshell itself.

Change-Id: I8edc2b9e9efa34402da2bb7cca15a3ae647266fa
Flag: com.android.wm.shell.enable_auto_task_stack_controller
Bug: 384082238 
Test: make