Lock down RemoteInputConnectionImpl#getInputConnection()

This is a mechanical refactoring CL that locks down

  RemoteInputConnectionImpl#getInputConnection()

as a private method.

This is supposed to be helpful to to avoid future misuse of raw
InputConnection instance outside RemoteInputConnectionImpl.

Note that InputMethodManager#isAcceptingText() remains to have the
same observable behavior in this CL.  The key fact is that the
following two fields are updated in an atomic way.

  * RemoteInputConnectionImpl#mInputConnection
  * RemoteInputConnectionImpl#mFinished

Bug: 192412909
Test: presubmit
Change-Id: Ic5fbc6213ad62df95fc0b7eef18bab1fd9fbdbf1