Log errors if RemoteInputConnection is used after IMS#onDestroy()

This CL consolidates weak-ref handlings in RemoteInputConnection so
that IME developers can find more useful messages in logcat when they
are using RemoteInputConnection after InputMethodService#onDestroy().

RemoteInputConnection has been designed to not hold a strong reference
to its parent InputMethodService instance.
While IME developers can manually keep a strong reference to
(Remote)InputConnection object that they obtained from
InputMethodService#getCurrentInputConnection(), doing so does not
prevent the system from garbage-collecting InputMethodService
instance.

With this CL, an error message will be shown in logcat if somehow
RemoteInputConnection object is still touched after InputMethodService
is destroyed, which hopefully can help IME developers realize they
might be doing something unexpected.

Other than showing error messages in logcat, there is no behavior
change in this CL.

Bug: 192412909
Bug: 194110780
Test: atest CtsInputMethodTestCases
Change-Id: I514630d27c8953f62fdb34cd3133a662b3fbbf76