LastFmServiceConfig: prompt to save credentials in ASCII even if KWallet is disabled

I believe there is inconsistency in KWallet documentation/behaviour that
lead to assumptions not being fulfilled in credential-saving code. Lessen
the assumptions.

BUGFIXES:
 * Definitely fix a bug where storing Last.fm credentials in plain-text
   isn't offered when KWallet is disabled. (BR 315306)

@Vedant, you might need to incorporate this fix to your
https://git.reviewboard.kde.org/r/110426/

BUG: 315306
FIXED-IN: 2.8
CCBUG: 322964
CCMAIL: Vedant Agarwala <vedant.kota@gmail.com>