When resuming the activity check for uninstall/install in the mean time.

Fixes #854.

There is a bunch of UI code which runs in `onResumeFragments()`. However
it all depends on the `app` variable in `AppDetails` being up to date.
There is a number of things which could've changed since last refresh,
the most notable of which is that the app may have been
installed/uninstalled. Thus, this particular change checks only for a
difference in installed state. However it could equally do a deeper
comparison between the old and new `App` objects if it becomes aparant
that other state becomes stale between pause and resume in the future.

It is true that this requires a database query to be run each time the
activity is resumed, but it doesn't seem there is much else we can do
to prevent bugs relating to stale state.

app/src/main/java/org/fdroid/fdroid/AppDetails.java

@@ -432,6 +432,15 @@ public class AppDetails extends AppCompatActivity {
                 myAppObserver);
     }
 
+    @Override
+    protected void onResume() {
+        App newApp = AppProvider.Helper.findHighestPriorityMetadata(getContentResolver(), app.packageName);
+        if (newApp.isInstalled() != app.isInstalled()) {
+            setApp(newApp);
+        }
+        super.onResume();
+    }
+
     @Override
     protected void onResumeFragments() {
         // Must be called before super.onResumeFragments(), as the fragments depend on the active