Switch BackgroundOnlyTheme from Holo to Material

I verified that all associated activities look okay.
* ExportVCardActivity
* ImportVCardActivity
* CancelActivity
* NfcImportVCardActivity isn't affected. This activity is compeltely invisible.
* SelectAccountActivity isn't affected in practice. This activity doesn't
  appear to ever be used. SelectAccountDialogFragment is always used first.

Screenshots in comments

Bug: 18024237
Change-Id: Ia5c13e41e60e89b40e75133f091fbf2cf1a4e05a