Allow back predict when no more activity below adjacnet task fragment.

For adjacent case, if both TaskFragment has only one activity left, then
even if they do not match shouldMoveTaskToBack, it can still predict for
previous task.

Bug: 311336844
Test: launch sample app from widget, verify both side can be predict for
back-to-home if no more activities below the task.

Change-Id: If22e4d71e19db8b5605214b717e334123970569f