Add rtl::OUString::toUtf8()

There is rtl::OUStringToOString() already to do OUString to OString conversion
using UTF-8 encoding on a best effort basis.  However multiple modules have
code where we assume that such a conversion is perfect. Add a new method that
asserts such an expected success instead of duplicating it at multiple places.

Change-Id: I0e55b53f558df82b67af6a463c8144655cf0ca74