Fix the string shown when authenticating the user.

The string for "Trying to authenticate with " dropped the whitespace
when appending the account name, which made the progress bar show
up with "Trying to authenticate withuser@chromium.org".

This changes it to use proper Android string formatting to insert the
account name at the right place.
Merge pull request #2 from curinir/fix-authenticate-string

Fix the string shown when authenticating the user.