Add support for preferred root task that can be set via launch params.

- This helps form factors to override the task launches and place them
  in a task stack container as desired by OEM/form-factor specific
  policy.

Flag: EXEMPT BUGFIX (doesn't affect existing code paths and used on any
      form-factor other than automotive)
Bug: 433539393
Test: atest LaunchParamsControllerTests

DISABLE_TOPIC_PROTECTOR

Change-Id: Iab675a09b7c23283ab49fd9ceffa4b8720527a8b