Potential issue with QuantileStateCoder

[https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ApproximateQuantiles.java#L687](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ApproximateQuantiles.java#L687)

The line above encodes the QuantileState buffers.size() as if it's numBuffers. This seems wrong since before buffers are full, buffers.size() is not equal to numBuffers. One thing I suspect will happen is that, if we serialize before buffer is full, it will effectively reduce the number of buffers we maintain after deserialization.

Imported from Jira [BEAM-4667](https://issues.apache.org/jira/browse/BEAM-4667). Original Jira may contain additional context.
Reported by: zhihengh.