Introduce a transition flag for background launched activity

testPI_onlyCreatorAllowsBALwithOptIn_isStartedInBackground() failed on wm_shell build because ScalableUI handled the background activity launch transition by moving the task to front.

To fix it, this CL introduced a new transition flag and set it in the transition when launching background activity. As a result, ScalableUI will be able to decide whether it should move the task to front or not by checking the transition flag.


Bug: 390565578
Test: manual
Flag: EXEMPT bug fix
Change-Id: I1ee0e5643e450defb97ffff121cd45dd1e630d77