Introduce IInputMethodInvoker

This CL introduces a wrapper class IInputMethodInvoker to consolidate
RemoteException handlings out from InputMethodManagerService around
IInputMethod.

Basically there should be no observable behavior change, except that
we logcat messages upon RemoteException when issuing IInputMethod IPCs
are now shown only when

  InputMethodManagerService#DEBUG

is configured to be true or the exception is not DeadObjectException.

Hopefully we can still easily find that the IME was being killed from
other messages in logcat.

Bug: 215609403
Test: presubmit
Change-Id: Id636d35f3b82a6376c9f370e2f65692f28e545eb