We are developing a library that will be used both in JVM and Android. And now we need to implement trial period notification. In other platforms (.Net and Mac) showing a popup message box was enough. But so far I cannot find a way to do this for Android.
The problem is that to show some message (toast or AlertDialog), I need to have an actual context. But since our library doesn't contain UI elements and not related to UI in any way we are not asking a user to pass context when creating out components.
To be short:
It appears that you really can't show neither Toast nor AlertDialog without having a context. The most close variant is to provide some interface in your library that user should implement.