Remove a redundant check from IMM#isAcceptingText()

This is a follow up CL to my previous CL [1], which rewrote

  InputMethodManager#isAcceptingText()

with a newly introduced internal method

  RemoteInputConnectionImpl#isFinished().

What I overlooked was the fact that

  mServedInputConnection.isFinished()

is guaranteed to return "true" while

  InputMethodManager#mServedInputConnection

is nonnull.  Let's simplify the logic.

This is a safe clean up.  There must be no observable behavior change.

 [1]: Ic5fbc6213ad62df95fc0b7eef18bab1fd9fbdbf1
      d1ff9740

Bug: 234882948
Test: presubmit
Change-Id: Icee431b05108aa607ea0a56c683a361399b7583e