Scan for new books more often.

Audiobooks are additionally rescanned when:
- the application UI is shown - this should help if the ContentObserver
  is not called after files are changed. Hopefully this helps with
  issues like #17,
- if no books are found during the initial scan rescan in 10 seconds
  which may help on devices where Homer Player is started before an SD
  card with audiobooks is attached (see #12).

app/src/main/java/com/studio4plus/homerplayer/model/AudioBookManager.java

@@ -1,5 +1,7 @@
 package com.studio4plus.homerplayer.model;
 
+import android.os.Handler;
+import android.os.Looper;
 import android.support.annotation.MainThread;
 import android.support.annotation.NonNull;
 
@@ -17,6 +19,7 @@
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
+import java.util.concurrent.TimeUnit;
 
 import javax.inject.Inject;
 
@@ -30,13 +33,13 @@ public class AudioBookManager {
     private final Storage storage;
     private AudioBook currentBook;
     private boolean isInitialized = false;
+    private boolean isFirstScan = true;
 
     @Inject
     @MainThread
     public AudioBookManager(EventBus eventBus, FileScanner fileScanner, Storage storage) {
         this.fileScanner = fileScanner;
         this.storage = storage;
-        scanFiles();
         eventBus.register(this);
     }
 
@@ -109,6 +112,19 @@ public void onException(@NonNull Throwable t) {
 
     @MainThread
     private void processScanResult(@NonNull List<FileSet> fileSets) {
+        if (isFirstScan && fileSets.isEmpty()) {
+            // The first scan may fail if it is just after booting and the SD card is not yet
+            // mounted. Retry in a while.
+            Handler handler = new Handler(Looper.myLooper());
+            handler.postDelayed(new Runnable() {
+                @Override
+                public void run() {
+                    scanFiles();
+                }
+            }, TimeUnit.SECONDS.toMillis(10));
+        }
+        isFirstScan = false;
+
         // This isn't very efficient but there shouldn't be more than a dozen audio books on the
         // device.
         List<AudioBook> booksToRemove = new ArrayList<>();

app/src/main/java/com/studio4plus/homerplayer/ui/UiControllerMain.java

@@ -65,6 +65,7 @@ void onActivityCreated() {
 
     void onActivityStart() {
         Crashlytics.log("activity start");
+        audioBookManager.scanFiles();
         maybeSetInitialState();
     }