Base class for single-track encoders, coordinated by a MediaEncoderEngine. For the lifecycle of this class, read comments in the engine class. This class manages a background thread and streamlines events on this thread which we call the EncoderThread: 1. When prepare is called, we call onPrepare on the encoder thread. 2. When start is called, we call onStart on the encoder thread. 3. When notify is called, we call onEvent on the encoder thread. 4. After starting, encoders are free to acquire an input buffer with tryAcquireInputBuffer or acquireInputBuffer. 5. After getting the input buffer, they are free to fill it with data. 6. After filling it with data, they are required to call encodeInputBuffer for encoding to take place. 7. After this happens, or at regular intervals, or whenever they want, encoders can then call drainOutput with a false parameter to fetch the encoded data and pass it to the engine (so it can be written to the muxer). 8. When stop is called - either by the engine user, or as a consequence of having called requestStop - we call onStop on the encoder thread. 9. The onStop implementation should, as fast as possible, stop reading, signal the end of input stream (there are two ways to do so), and finally call drainOutput for the last time, with a true parameter. 10. Once everything is drained, we will call onStopped, on a unspecified thread. There, subclasses can perform extra cleanup of their own resources. For VIDEO encoders, things are much easier because we skip the whole input part. See description in VideoMediaEncoder. MAX LENGTH CONSTRAINT For max length constraint, it will be checked automatically during drainOutput, OR subclasses can provide an hint to this encoder using notifyMaxLengthReached. In this second case, we can request a stop at reading time, so we avoid useless readings in certain setups (where drain is called a lot after reading). TIMING Subclasses can use timestamps (in microseconds) in any reference system they prefer. For instance, it might be the nanoTime reference, or some reference provided by SurfaceTextures. However, they are required to call notifyFirstFrameMillis and pass the milliseconds of the first frame in the currentTimeMillis reference, so something that we can coordinate on.