Check that the Google Play Store exists on device

Fixes #3

extension-iap/src/java/com/defold/iap/IapGooglePlay.java

@@ -155,10 +155,24 @@ public class IapGooglePlay implements Handler.Callback {
         this.autoFinishTransactions = autoFinishTransactions;
     }
 
+    private static boolean isPlayStoreInstalled(Context context){
+        try {
+            context.getPackageManager().getPackageInfo("com.android.vending", 0);
+            return true;
+        } catch (PackageManager.NameNotFoundException e) {
+            return false;
+        }
+    }
+
     private void init() {
         // NOTE: We must create Handler lazily as construction of
         // handlers must be in the context of a "looper" on Android
 
+        if (!isPlayStoreInstalled(activity)) {
+            Log.e(TAG, "Unable to find Google Play Store (com.android.vending)");
+            return;
+        }
+
         if (this.initialized)
             return;