Add icon-bg and icon-fg to fix_args + adds a missing folder in webview dir template (kivy#2633)

* Add icon-bg and icon-fg to fix_args

Fix the icon-bg and icon-fg arguments and also create the folder
mipmap-anydpi-v26 if it doesn't exists before trying to create a file
inside.

* Use existing ensure_dir function to create anydpi_dir

* Add mipmap-anydpi-v26 to bootstrap template

This makes available the directory in the final template so it's not
needed to check the directory exists.

Author: danigm

pythonforandroid/bootstraps/webview/build/src/main/res/mipmap-anydpi-v26/.gitkeep

@@ -990,7 +990,8 @@ def _fix_args(args):
         """
 
         fix_args = ('--dir', '--private', '--add-jar', '--add-source',
-                    '--whitelist', '--blacklist', '--presplash', '--icon')
+                    '--whitelist', '--blacklist', '--presplash', '--icon',
+                    '--icon-bg', '--icon-fg')
         unknown_args = args.unknown_args
 
         for asset in args.assets: