My problem was two-fold.
1) Instead of using com.google.api.client.http.javanet.NetHttpTransport, I used com.google.api.client.http.apache.ApacheHttpTransport and saw approximately a 5x speed increase.
This also fixed a separate problem where I got socket errors trying to using Google Cloud Endpoints (with a NetHttpTransport) immediately after doing a GCS transfer.
2) Trying to run the upload while debugging with ADB accounted for the rest of the difference. Running in non-debug mode with ApacheHttpTransport brought my transfer speeds up to about 1.4 Mbps, where my local network has about a 1.8 Mbps capacity.
By the way, I attempted to use JetS3t but ran into this exact problem: IncompatibleClassChangeError exception is thrown when using JetS3t on Android