Android requires the following folders:

Assets are copied directly into these folders using file names containing only lowercase letters, numbers (0-9), ".", and "_" (e.g., "my_header_image_14.png"). The file name used for the different versions of the image must be exactly the same in all folders. No sub-directories are allowed.

Other folders, such as drawable-nodpi, may also be appropriate for your project. For more information, see https://developer.android.com/guide/practices/screens_support.html, sections Range of Screens Supported and Using Configuration Qualifiers.