How do I create a voice assistant app for Android?
Probably the easiest way is by using Pandorabots CallMom™. I believe Pannous was involved in the development of CallMom, and offers several APIs on Mashape. Pandorabots also has a new API available at chatbots.io.
Guile Lindroth seems to be close to releasing a mobile version of his customizable Guile3D Denise virtual assistant, though not a true mirror or extension of the popular desktop assistant, but more like an additional product.
Good open source options include PHP based Program O as well as Bruce Wilcox‘ award winning ChatScript (which has again, just won the Loebner Prize for 2014).