Kivy: use the same password for all wallets

When the app is started, the password is checked against all
wallets in the directory.

If the test passes:
 - subsequent wallet creations will use the same password
 - subsequent password updates will be performed on all wallets
 - wallets that are not storage encrypted will encrypted
   on the next password update (even if they are watching-only)

This behaviour is restricted on Android, with a 'single_password' config variable.
Wallet creation without password is disabled if single_password is set

run_electrum

@@ -317,6 +317,7 @@ def main():
             'verbosity': '*' if build_config.DEBUG else '',
             'cmd': 'gui',
             'gui': 'kivy',
+            'single_password':True,
         }
     else:
         config_options = args.__dict__