Ensure IC#finishComposingText() is called on the correct Handler.

This attempts to reland previously reverted CLs [1][2] due to an
unexpected regression (Bug 27824691).

The Bug 27868748 we want to address by this CL is that currently
InputConnection#finishComposingText() can be called on the root view's
Handler no matter what Handler is associated with
ControlledInputConnectionWrapper.  Actually the root cause of
Bug 6789252 is the same, but there we worked around it by not calling
InputConnection#finishComposingText() in certain situations [3].
With this CL we should be able to logically revert that workaround.

This CL also removes redundant IMM#mServedInputConnection.  This is safe
because the following two fields have the same lifetime.
 - InputMethodManager#mServedInputConnection
 - InputMethodManager#mServedInputConnectionWrapper
We do not need to maintain both of them.  This also allows us to use a
strong refecente in IInputConnectionWrapper#mInputConnection instead of
a WeakReference.  To understand why this is safe, we need to understand
how things previously worked, which is as follows:

  1. InputMethodManager#mServedInputConnection becomes non-null.
    -> IInputConnectionWrapper#mInputConnection.get() is guaranteed to
       be alive.
  2. InputMethodManager#mServedInputConnection becomes null or another
     object.
    -> IInputConnectionWrapper#mInputConnection.get() may not be alive.

Since we know exactly when InputMethodManager#mServedInputConnection is
updated, in theory we do not need to use WeakReference here, and
with this CL we do not use WeakReference anymore.  Actually the initial
commit [1] accidentally removed the last strong reference to the active
InputConnection and WeakReference could be null at any time, which was
what we observed in Bug 27824691.

  [1]: I1181e067aa5bedbdf0c7ec1bcec479257aea511c
       afb6558c
  [2]: Ibe94f115e607a198d12ecd3d4e4f91a7d9469c98
       16e2c7b5
  [3]: I66f51da1299532793ef8fa700f35b0811670f235
       4e5184f9

Bug: 27868748
Change-Id: If2a03bc84d318775fd4a197fa43acde086eda442