Make some inner classes static

This avoids an implicit this field and can help GC.  Found via
error-prone.

app/src/main/java/de/danoeh/antennapod/activity/FeedInfoActivity.java

@@ -379,7 +379,7 @@ public class FeedInfoActivity extends AppCompatActivity {
         }
     }
 
-    private class ApplyToEpisodesDialog extends ConfirmationDialog {
+    private static class ApplyToEpisodesDialog extends ConfirmationDialog {
 
         private final Feed feed;
         private final boolean autoDownload;

app/src/main/java/de/danoeh/antennapod/adapter/itunes/ItunesAdapter.java

@@ -162,7 +162,7 @@ public class ItunesAdapter extends ArrayAdapter<ItunesAdapter.Podcast> {
     /**
      * View holder object for the GridView
      */
-    class PodcastViewHolder {
+    static class PodcastViewHolder {
 
         /**
          * ImageView holding the Podcast image

app/src/main/java/de/danoeh/antennapod/fragment/DownloadsFragment.java

@@ -78,7 +78,7 @@ public class DownloadsFragment extends Fragment {
         viewPager.setCurrentItem(lastPosition);
     }
 
-    public class DownloadsPagerAdapter extends FragmentPagerAdapter {
+    public static class DownloadsPagerAdapter extends FragmentPagerAdapter {
 
         Resources resources;
 

core/src/main/java/de/danoeh/antennapod/core/service/download/DownloadService.java

@@ -950,7 +950,7 @@ public class DownloadService extends Service {
      * <p/>
      * Currently, this handler only handles FeedMedia objects, because Feeds and FeedImages are deleted if the download fails.
      */
-    private class FailedDownloadHandler implements Runnable {
+    private static class FailedDownloadHandler implements Runnable {
 
         private DownloadRequest request;
         private DownloadStatus status;

core/src/main/java/de/danoeh/antennapod/core/service/download/HttpDownloader.java

@@ -314,7 +314,7 @@ public class HttpDownloader extends Downloader {
         }
     }
 
-    private class BasicAuthorizationInterceptor implements Interceptor {
+    private static class BasicAuthorizationInterceptor implements Interceptor {
 
         private DownloadRequest downloadRequest;
 

core/src/main/java/de/danoeh/antennapod/core/util/syndication/HtmlToPlainText.java

@@ -40,7 +40,7 @@ public class HtmlToPlainText {
     }
 
     // the formatting rules, implemented in a breadth-first DOM traverse
-    private class FormattingVisitor implements NodeVisitor {
+    private static class FormattingVisitor implements NodeVisitor {
 
         private StringBuilder accum = new StringBuilder(); // holds the accumulated text