Simplify RemoteInputConnectionImpl#finishComposingText*()

With my previous CLs [1][2], we can generally assume the following
things.

 - InputConnection#closeConnection() always gets called when it
   becomes invalidated, including the case when the IME has switched
   to another IME client.

 - Each implementation of InputConnection#closeConnection() is
   expected to do appropriate clean up including finishing composing
   text. {Base,Editable}InputConnection actually do this correctly.

With that it should be safe for #finishComposingText*() to follow the
same pattern about isActive().

 [1]: I234309c5880c9fe0b299b8bd0f8862796d4dda0d
      9f9afe52
 [2]: If2a03bc84d318775fd4a197fa43acde086eda442
      aaa38c9f

Bug: 35301295
Bug: 291826769
Test: presubmit
Change-Id: If913701bf8555f5d76fceb272e38a99f5e243bbe