Revert "InputConnectionWrapper never supports null target."

This reverts commit 90bd3636.

Seems that the semantics of InputConnectionWrapper#setTarget() is more
complicated than I thought.  At least the following cases have worked
fine.

  case 1:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(null, false);
    wrapper.SetTarget(ic);
    ...

  case 2:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(null, true);
    wrapper.SetTarget(ic);
    ...

  case 3:
    InputConnectionWrapper wrapper =
            new InputConnectionWrapper(ic, true);
    wrapper.SetTarget(null);
    wrapper.SetTarget(ic2);
    ...

The previous code did not intended to break existing code.  Let's revert
it we decide how to deal with above cases.

Bug: 27407697
Change-Id: I8bc84d484ab0b27a02e74f11110430f70646e69a