With the latest major release from December 2016, K-9 Mail dropped support for APG as an OpenPGP provider. The drop-in replacement is OpenKeychain, which has been actively maintained by Dominik Schürmann and myself since it was originally forked from APG in 2012.
There are two reasons for this decision:
Firstly, development of APG ceased since its last release in March 2014. Moving forward with PGP/MIME support required a number of changes to the crypto provider API, rendering APG incompatible with K-9 Mail in newer versions.
The second reason, however, is the more important one: Three years is a very long time for security-relevant software, and no release during this time means no bugs were fixed. Meanwhile, OpenKeychain had a security audit in 2015 that yielded a number of issues. All of these issues were fixed in OpenKeychain, but are still present in APG due to their shared development history.