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 in a form like this: `/SELECTED_LIBRARY_NAME/Camera Uploads/photos_name.xxx`. 

Note that, the `Camera Uploads` folder will be automatically created itself when first setup or deleted. For an extreme circumstance, if the library was deleted by the user, the app will stop uploading photos and ask user to choose another library to continue.
this value is used to filter the latest camera photo so that we can upload new photos taken by camera. If set the value to null, we will always get the oldest one, and the new photo never be uploaded.