Use WeakReference#refersTo() when possible

For a given WeakReference<T> ref,

  ref.refersTo(obj)

is more efficient than

  ref.get() == obj.

This CL applies the above rule to

  RemoteInputConnectionImpl

for a better performance.

This is mechanical refactoring.  There must be no observable behavior
change.

Bug: 182259171
Test: presubmit
Change-Id: Iddf73b0fcb16a14f6c024da4a7db136d4fea4900