title: getCurrentActivity in ReactContextBaseJavaModule returns null
body: <p>I'm coding a native Android module with React Native 0.46 and I have trouble getting the current activity instance from the module.</p>
  
  <p>I have a class extending ReactContextBaseJavaModule, which contains a built-in getCurrentActivity() method. However, each time I call this method, I get a null.</p>
  
  <p>I think it's because I'm calling this in my module constructor, which is executed at the start of the Android application, maybe before an Activity is instantiated ?</p>
  
  <p>If you guys know a clean way to access the current Activity instance from within the module (without having to store an Activity instance at some point and passing it around, if possible), I'll be glad !</p>