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/res/values/strings.xml

@@ -195,6 +195,7 @@
     <string name="wrong_passphrase">Wrong passphrase.</string>
     <string name="set_a_passphrase">Set a passphrase first.</string>
     <string name="no_filemanager_installed">No compatible file manager installed.</string>
+    <string name="filemanager_no_write">The file manager does not support saving.</string>
     <string name="passphrases_do_not_match">The passphrases didn\'t match.</string>
     <string name="passphrase_must_not_be_empty">Please enter a passphrase.</string>
     <string name="passphrase_for_symmetric_encryption">Symmetric encryption.</string>