The documentation for ANDROID_EXTRA_PLUGINS says:

"Specifies a path to C++ plugins or resources that your application has to bundle but that cannot be delivered through the assets system, such as QML plugins. With this variable, androiddeployqt will make sure everything is packaged and deployed properly."

However, it seems that when run, all that happens is that the plugins in this directory are copied to the apk. Any dependencies the plugins have do not appear to be resolved. So, if, for example, a plugin uses a Qt library that isn't used by the main application, it will be missing from the apk.