Show error dialog only when 'ask first' is not set

For call to USSD code, if mCalls contain current call, that means
'ask first' is enabled in which case InCallUi will take care of
displaying error dialog and hence skip dialog display here in order
to avoid duplication of dialogs.

Test: manually test
Bug: 38485469
Change-Id: I54b68d3f44d92e40be1a48f64b4ec84d1971dcd6