Android: properly set default dir for the document browser

The default dir had been changing by modifying the "local documents"
provider; this is wrong because there is already a document provider
to browser de internal storage.

Change-Id: I08677283100162aaeced622153131f89abc2f1af
Reviewed-on: https://gerrit.libreoffice.org/16244Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>

android/source/src/java/org/libreoffice/storage/DocumentProviderFactory.java

@@ -105,7 +105,7 @@ public final class DocumentProviderFactory {
      * @return default provider.
      */
     public IDocumentProvider getDefaultProvider() {
-        return providers[0];
+        return providers[1];
     }
 
     public Set<OnSharedPreferenceChangeListener> getChangeListeners() {

android/source/src/java/org/libreoffice/storage/local/LocalDocumentsDirectoryProvider.java

@@ -28,7 +28,8 @@ public class LocalDocumentsDirectoryProvider extends LocalDocumentsProvider {
     @Override
     public IFile getRootDirectory() {
         File documentsDirectory = new File(
-                Environment.getExternalStorageDirectory(), "");
+                Environment.getExternalStorageDirectory(), "Documents");
+        documentsDirectory.mkdirs();
         return new LocalFile(documentsDirectory);
     }