Running Espresso test on a specific Android application from outside that application

I would like to write an Espresso test that I can run on an application from outside that application. I don't have much experience with Android, but, as I understand it, a UI test is generally written and executed from inside the project of the application that it is supposed to test. For instance, according to the file explorer in Android Studio, the java files for a test application I was developing are stored in com.package.example.name, and the java files for the UI test are stored in com.package.example.name (androidTest).

I want to develop a UI test for an application (Application A) for which I do not have the project files. As I do not have the project for Application A, I cannot develop the test for Application A from inside that same project like I normally would. Thus, I have to somehow create and run it from outside.

My initial thought was to create a test application (Application B) from scratch and add a button to it that, when pressed, will open Application A. I would develop an Espresso test from inside Application B, have it press that button to launch Application A, and then have it follow other Espresso instructions I would develop by viewing the R.ids of Application A's UI elements through uiautomatorviewer.bat, included in the Android SDK tools.

At least to me, this seemed like it would work just fine in theory, so I went forward with the plan. However, when the button inside Application B is pressed by the UI test and Application A launches, the test cannot find an activity to continue testing on. No error is explicitly thrown; Logcat prints messages like "W/RootViewPicker﹕ No activity currently resumed - waiting: 2000ms for one to appear," repeatedly, where 2000ms increases as time goes on.

Historically, I have not had any problems when running Espresso tests across activity changes. I assume this issue has something to do with the fact that the activity being launched exists outside of the project where the Espresso test is based.

If anyone knows of a way to get an Espresso test to run successfully on an application when based in a project located outside of that application, I'd appreciate any advice you may be able to offer. I think I'm on the right track, but I don't know for sure. Thanks!