Clarify how Handler is used in IC#getHandler()

Since its beginning [1] the system has been using

  InputConnection#getHandler()

only for retrieving Looper associated with it.  Although it has been
intentional, it'd be probably helpful for app developers if we can
clarify a bit more on this, because most likely the current behavior
is not going to change.

This CL also annotates the API with @Nullable as it should have been.

 [1]: Id9e579bb3e2966986cdcb1c34bc8cacfeca2e1a9
      612cce92

Bug: 26945674
Fix: 195434171
Test: presubmit
Change-Id: I14910c6f0cc43feec505b2951625d0c00203ac92