tdf#87434: android: system back key to go one level up

Added an additional check so back has to be pressed twice on the root
folder to actually leave the application. It's a check seen in many
other apps.

Change-Id: I26827113a41070aa8188fa616ba8fe53742329b3
Reviewed-on: https://gerrit.libreoffice.org/16245Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>

android/source/res/values/strings.xml

@@ -14,6 +14,7 @@
     <string name="about_license">Show License</string>
     <string name="about_notice">Show Notice</string>
     <string name="about_moreinfo">More Info</string>
+    <string name="back_again_to_quit">Press back again to quit</string>
 
     <string name="browser_app_name">LibreOffice Browser</string>
     <string name="menu_search">Search</string>

android/source/src/java/org/libreoffice/ui/LibreOfficeUIActivity.java

@@ -20,6 +20,7 @@ import android.graphics.drawable.BitmapDrawable;
 import android.net.Uri;
 import android.os.AsyncTask;
 import android.os.Bundle;
+import android.os.Handler;
 import android.preference.PreferenceManager;
 import android.support.v4.widget.DrawerLayout;
 import android.support.v7.app.ActionBar;
@@ -96,6 +97,7 @@ public class LibreOfficeUIActivity extends ActionBarActivity implements ActionBa
     ListView lv;
 
     private final LOAbout mAbout;
+    private boolean canQuit = false;
 
     public LibreOfficeUIActivity() {
         mAbout = new LOAbout(this, true);
@@ -197,6 +199,31 @@ public class LibreOfficeUIActivity extends ActionBarActivity implements ActionBa
         drawerLayout.closeDrawer(drawerList);
     }
 
+    @Override
+    public void onBackPressed() {
+        if (!currentDirectory.equals(homeDirectory)) {
+            // navigate upwards in directory hierarchy
+            openParentDirectory();
+        } else {
+            // only exit if warning has been shown
+            if (canQuit) {
+                super.onBackPressed();
+                return;
+            }
+
+            // show warning about leaving the app and set a timer
+            Toast.makeText(this, R.string.back_again_to_quit,
+                    Toast.LENGTH_SHORT).show();
+            canQuit = true;
+            new Handler().postDelayed(new Runnable() {
+                @Override
+                public void run() {
+                    canQuit = false;
+                }
+            }, 3000);
+        }
+    }
+
     @Override
     public void onCreateContextMenu(ContextMenu menu, View v,
             ContextMenuInfo menuInfo) {