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