track AppDetails visibility to improve Notification UX

If AppDetails is visible, then it'll automatically launch the install
process, and there is no need to put up a "Tap to install" notification.

And of course, whenever I search stackoverflow, I find an answer from
@commonsguy :)
https://stackoverflow.com/questions/18038399/how-to-check-if-activity-is-in-foreground-or-in-visible-background/18469643#18469643

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

@@ -110,6 +110,15 @@ public class AppDetails extends AppCompatActivity {
     private FDroidApp fdroidApp;
     private ApkListAdapter adapter;
 
+    /**
+     * Check if {@code packageName} is currently visible to the user.
+     */
+    public static boolean isAppVisible(String packageName) {
+        return packageName != null && packageName.equals(visiblePackageName);
+    }
+
+    private static String visiblePackageName;
+
     private static class ViewHolder {
         TextView version;
         TextView status;
@@ -437,6 +446,7 @@ public class AppDetails extends AppCompatActivity {
         if (DownloaderService.isQueuedOrActive(activeDownloadUrlString)) {
             registerDownloaderReceivers();
         }
+        visiblePackageName = app.packageName;
     }
 
     /**
@@ -458,6 +468,7 @@ public class AppDetails extends AppCompatActivity {
     @Override
     protected void onPause() {
         super.onPause();
+        visiblePackageName = null;
         // save the active URL for this app in case we come back
         PreferencesCompat.apply(getPreferences(MODE_PRIVATE)
                 .edit()

app/src/main/java/org/fdroid/fdroid/installer/InstallManagerService.java

@@ -175,7 +175,11 @@ public class InstallManagerService extends Service {
                 // TODO these need to be removed based on whether they are fed to InstallerService or not
                 Apk apk = ACTIVE_APKS.remove(urlString);
                 ACTIVE_APPS.remove(apk.packageName);
-                notifyDownloadComplete(apk, urlString);
+                if (AppDetails.isAppVisible(apk.packageName)) {
+                    cancelNotification(urlString);
+                } else {
+                    notifyDownloadComplete(apk, urlString);
+                }
                 unregisterDownloaderReceivers(urlString);
             }
         };
@@ -186,6 +190,9 @@ public class InstallManagerService extends Service {
                 Apk apk = ACTIVE_APKS.remove(urlString);
                 ACTIVE_APPS.remove(apk.packageName);
                 unregisterDownloaderReceivers(urlString);
+                if (AppDetails.isAppVisible(apk.packageName)) {
+                    cancelNotification(urlString);
+                }
             }
         };
         localBroadcastManager.registerReceiver(startedReceiver,
@@ -269,6 +276,15 @@ public class InstallManagerService extends Service {
         nm.notify(downloadUrlId, builder.build());
     }
 
+    /**
+     * Cancel the {@link Notification} tied to {@code urlString}, which is the
+     * unique ID used to represent a given APK file. {@link String#hashCode()}
+     * converts {@code urlString} to the required {@code int}.
+     */
+    private void cancelNotification(String urlString) {
+        notificationManager.cancel(urlString.hashCode());
+    }
+
     /**
      * Install an APK, checking the cache and downloading if necessary before starting the process.
      * All notifications are sent as an {@link Intent} via local broadcasts to be received by