Merge "media: clarify that flush() is not supported for encoders." into tm-dev am: 381a3b66

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/18590094



Change-Id: Ibd253b0db18de6ee5e13bb1bd27920080ab35e29
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>