Wouldn't it make more sense to bind Nim to Android NDK? I think you would have a hard time to call Android UI libs through jnim, unless you provide a thick layer. There is also NativeActivitywhich enables you to write purely native apps.
It is possible to extend jnimin such a way that no bindings should be defined by hand. When importing a java class, jnim could read class info in compile time and generate all the needed bindings. In other words its possible to make nim "automagically"compatible with Java with no hand-written bindings. As a side note nimx uses jnim on Android to get pixel density.