Merge "media: clarify that flush() is not supported for encoders." into tm-dev am: 381a3b66 am: 383e903a am: 72ef4e5e am: 708219a4

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



Change-Id: I2eb9f1e4f7d60a3cb1ea71b042dfbb6b322736b5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>