FlacEncoder: handle EOS properly. am: f4db80421f am: 6eeeed94b1 am: 6eb7c51e78

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/av/+/12104883

Change-Id: I27dee2309d0c5febeff8e8a4251b325472b540e2