use Camera Uploads folder to upload photos

When user turns on the camera photo upload service, the app will ask the user to choose a library first. Before I used SeafilePathChooserActivity to let user choose a library, but with this option, user can navigate into the library and choose folders or sub-folders, which seems strange because we only upload photos under the root of the selected library. So I changed to use SeafileLibraryChooserActivity class instead, in that way user only can choose a library, and photos will be uploaded into `/SELECTED_LIBRARY_NAME/Camera Uploads/`. 

Note that, the `Camera Uploads` folder will be automatically created itself when first setup or deleted. If the library was deleted by the user, the app will stop uploading and asks user to choose a new library