ExampleViewerActivity: Make the Toolbar's Up Button go back properly.

Override onOptionsItemSelected() so we can modify the Intent
used with navigateUpTo(), to specify FLAG_ACTVITY_CLEAR, so we
resume the parent activity instead of recreating it, just as if the
user had used the back button - which the user generally cannot
distinguish from the up button anyway.
We could instead have marked the parent activity as singleTop
in the manifest, but it seems nice to keep this parent activity
more flexible.

This should fix this bug:
#14

app/src/main/java/com/murrayc/galaxyzoo/app/ExampleViewerActivity.java

@@ -22,6 +22,8 @@
 import android.content.Intent;
 import android.os.Bundle;
 import android.support.v4.app.FragmentManager;
+import android.support.v4.app.NavUtils;
+import android.view.MenuItem;
 
 
 public class ExampleViewerActivity extends BaseActivity {
@@ -65,4 +67,33 @@ protected void onCreate(final Bundle savedInstanceState) {
 
         showUpButton();
     }
+
+    @Override
+    public boolean onOptionsItemSelected(final MenuItem item) {
+        // Handle presses on the action bar items
+        final int id = item.getItemId();
+
+        if (id == android.R.id.home) {
+            //The base class just uses NavUtils.navigateUpFromSameTask() but we want to make sure
+            //that the parent activity will be resumed instead of restarted,
+            //to make sure we go back to the correct help for the correct question,
+            //so we use FLAG_ACTIVITY_CLEAR_TOP.
+            //Alternatively, we could just mark QuestionHelpActivity as
+            //android:launchMode="singleTop" in the AndroidManifest.xml but it seems reasonable to
+            //use QuestionHelpActivity from other places one day.
+
+            //We can use this instead of more complex code, checking NavUtils.shouldUpRecreateTask(),
+            //because we know that our activities will never be opened from another app.
+            //See http://developer.android.com/training/implementing-navigation/ancestral.html.
+            final Intent upIntent = NavUtils.getParentActivityIntent(this);
+            if (upIntent != null) {
+                upIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
+            }
+
+            NavUtils.navigateUpTo(this, upIntent);
+            return true;
+        }
+
+        return super.onOptionsItemSelected(item);
+    }
 }