Consistently define plain string literals here, no OUStrings

cf. ef3fb927977681c3e5b5160e961e240063a8cb70 "svx: Use appropriate OUString
functions on string constants," 897f35006ea48b7b60b087a8f0b955c7f099af3a "fix
build," 3f66df98050175dfbab2026661c0cba734885e05 "Work around GCC bug"

Change-Id: Ifdfaad30cb59d54ac71bfe9347d29fb853551fd4