Work around std::wstring not being defined on Android.

Work around std::wstring not being defined on Android.

libstdc++ requires wchar.h (which bionic provides), but it also requires
additional functionality which bionic does not provide, thus, std::wstring
proper basically doesn't exist.

Provide a compatibility shim so we can keep API intact. This feature existed in
a similar guise in the past, but was removed in
d868c9945a188d6ad22e0b7d6d24ac7fca00ab4e.

Change-Id: I6cab6f41d04ad9dde97e3ce73506f9d8a42043fb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>