Simplify ResultCallbacks by not using WeakReference

This is a follow up CL to my previous CL [1], which introduced
ResultCallbacks in a bit conservative way, that is, to always wrap
Completable.* with WeakReference in case the target app fails to
respond for some reasons.

Using WeakReference would make sense if otherwise we would put more
stress on the memory than holding WeakReference object itself.  In our
case the expected memory stress on extending lifetime of Completable.*
objects is still comparable with the memory cost of holding
WeakReference objects instead.  Let's remove WeakReference from
ResultCallbacks in favor of simplicity.

This change should be completely transparent to other layers since
there is no change in the semantics (except for the lifetime itself of
Completable.* object, which is technically observable with APIs such
as java.lang.ref.Cleaner).

 [1]: Ic65a95eb5d0fd56f505a02fd9083bcf6694b6734
      f87f7508

Bug: 192412909
Test: atest -c CtsInputMethodTestCases
Change-Id: I43966494ccb4fceb8e7f8c60bd6ce3cc7fbfdd82