Add extra crash info for onHostPause

Summary: Several apps, (but most affected is Ads Manager) have seen crashes with the assertion on this line. Adding this extra info on these crashes can help us narrow down the root cause.

Differential Revision: D66980806

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -651,7 +651,10 @@ public void onUserLeaveHint(@Nullable Activity activity) {
   @ThreadConfined(UI)
   public void onHostPause(@Nullable Activity activity) {
     if (mRequireActivity) {
-      Assertions.assertCondition(mCurrentActivity != null);
+      Assertions.assertCondition(
+          mCurrentActivity != null,
+          "onHostPause called with null activity, expected: "
+              + mCurrentActivity.getClass().getSimpleName());
     }
     if (mCurrentActivity != null) {
       Assertions.assertCondition(