Improve file more, Part 1

- Use Uris where it makes sense, Use File class to clarify it's a file (and not whatever else a string could be)
- Show sdcard in side menu in storage API #665
- Propose filename with gpg ending when storing it using the storage API #665
- Don't show output dialog on Android 4.4 #665
- Only show filename on Android < 4.4 #665

TODO:
- File deletion for Android < 4.4
- Testing (especially with Android < 4.4)
- Batch-encryption
- UI
- Temporary content provider (see #665 discussion)

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/KeychainApplication.java

@@ -26,11 +26,9 @@ import android.graphics.drawable.Drawable;
 import android.os.Environment;
 
 import org.spongycastle.jce.provider.BouncyCastleProvider;
-import org.sufficientlysecure.keychain.helper.ContactHelper;
 import org.sufficientlysecure.keychain.util.Log;
 import org.sufficientlysecure.keychain.util.PRNGFixes;
 
-import java.io.File;
 import java.security.Provider;
 import java.security.Security;
 
@@ -70,8 +68,7 @@ public class KeychainApplication extends Application {
 
         // Create APG directory on sdcard if not existing
         if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
-            File dir = new File(Constants.Path.APP_DIR);
-            if (!dir.exists() && !dir.mkdirs()) {
+            if (!Constants.Path.APP_DIR.exists() && !Constants.Path.APP_DIR.mkdirs()) {
                 // ignore this for now, it's not crucial
                 // that the directory doesn't exist at this point
             }