Fix ambiguous String8 conversion

The MtpString type has implicit conversions to const char* as well as
std::string_view.

Explicitly convert to std::string_view when passing to String8 to
resolve overload ambiguity in the direction of the preferable call.

Test: compiles
Flag: EXEMPT infeasible
Bug:
Change-Id: Iea50eef3fcba628173fcec784401e8c0b867b90d