Update KEY_TASK_FRAGMENT_TOKEN to align WM Extensions

Update the naming to align
ActivityEmbeddingOptionsProperties#KEY_ACTIVITY_STACK_TOKEN,
or #setLaunchingActivityStack won't work.

Test: presubmit
Bug: 295993745
Change-Id: Ib657cc9fc3d62e1bca8b459b150cbd8593b865d0
1 file changed