vr: avoid missing std::char_traits<const T>

In newer versions of libc++, std::char_traits<T> is only defined for
character types (and not const/volatile-qualified character types). See
https://discourse.llvm.org/t/deprecating-std-string-t-for-non-character-t/66779.

Remove the cv qualifiers from CharT first. Keep qualifiers on
value_type and typedefs derived from it.

This library appears to be unused (see b/253514421#comment11), but it
still builds with a "checkbuild", so it blocks the libc++ update
(b/175635923).

Bug: 175635923
Test: make checkbuild
Test: m MODULES-IN-frameworks-native-libs-vr
Test: /data/nativetest64/pdx_tests/pdx_tests
Change-Id: I2bfa04ccd073f8701597be8618bb8b48eb695afe

Author: rprichard

libs/vr/libpdx/private/pdx/rpc/string_wrapper.h

@@ -17,12 +17,12 @@ namespace rpc {
 // C strings more efficient by avoiding unnecessary copies when remote method
 // signatures specify std::basic_string arguments or return values.
 template <typename CharT = std::string::value_type,
-          typename Traits = std::char_traits<CharT>>
+          typename Traits = std::char_traits<std::remove_cv_t<CharT>>>
 class StringWrapper {
  public:
   // Define types in the style of STL strings to support STL operators.
   typedef Traits traits_type;
-  typedef typename Traits::char_type value_type;
+  typedef CharT value_type;
   typedef std::size_t size_type;
   typedef value_type& reference;
   typedef const value_type& const_reference;