Android: Fix crash in main activity

android/app/src/main/java/com/github/stenzek/duckstation/AndroidHostInterface.java

@@ -54,6 +54,9 @@ public class AndroidHostInterface
         return mInstance != null;
     }
 
+    static public boolean hasInstance() {
+        return mInstance != null;
+    }
     static public AndroidHostInterface getInstance() {
         return mInstance;
     }

android/app/src/main/java/com/github/stenzek/duckstation/MainActivity.java

@@ -44,7 +44,7 @@ public class MainActivity extends AppCompatActivity {
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
-        if (!AndroidHostInterface.createInstance(this)) {
+        if (!AndroidHostInterface.hasInstance() && !AndroidHostInterface.createInstance(this)) {
             Log.i("MainActivity", "Failed to create host interface");
             throw new RuntimeException("Failed to create host interface");
         }