Fix several decoder bugs.

- NativeConverter should report unterminated character
  sequences to managed code.
- CharsetDecoderIcu should report those errors correctly
  from flush()
- InputStreamReader should check the return value of
  flush() and not ignore it altogether.

bug: 11665359
bug: 10729779

Change-Id: Ic678a645a4ef2b3fe70e2c900c0cd393679f1037