developer experience: on Android 10, when developer mode turned on, ask permission to manage AlphaWallet root folder

Android 10 can't create directories off root, hence αWallet can't create a top level AlphaWallet directory for users to through in test TS files.

We should

1. Modify the developer's guide to ask them to create a top-level AlphaWallet directory if there isn't one on the root level.

2. Modify αWallet Android so that it checks if such a folder exist - if it does, scan it for TokenScript.

3. Give user some visual hints that an AlphaWallet directory is found and is at work. e.g. having an extra menu item in "Settings" where, if the user clicks it, the user gets a list of TokenScript files in that directory, together with the status if they are recognised (in the case the user dropped a random XML file in it).

This is related to #1274