mik3y/usb-serial-for-android is a Java driver library that turns an Android device into a USB host so you can talk directly to Arduino and other USB serial hardware. It covers USB serial devices including CDC, FTDI, and Arduino, and the latest published release is 3.11.0.
This library is worth looking at on its own because the driver is implemented in pure Java. Android's USB host mode (Host Mode, OTG) is the feature that lets the device itself become the USB host and control attached peripherals, and this driver works on top of it with no root access, no ADK, and no special kernel driver. You can use it to perform reads and writes against a device directly and build your own protocol, and since both direct read/write and event-based read examples are included, it is easy to follow the communication structure as-is. The implementation language is Java, the license is MIT, and the star count is 5702.
That said, the changelog for 3.11.0 and any performance benchmarks are not included in the given record, so it is too early to judge what this version changed and how it performs in practice. The repository was created on June 30, 2013, and among the evaluation items momentum topped out at 1.0 out of 10, while no figures were given for the remaining items. For the purpose of communicating with an attached device it is close to an option worth trying, but the actual scope of the improvements in the new version can only be known by comparing it against the release text.