Remove invalid constexpr until we get to C++17.

frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp:236:16: error: constexpr variable cannot have non-literal type 'const android::(anonymous namespace)::(lambda at frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp:236:38)'
constexpr auto xmlDeleter<xmlChar> = [](xmlChar *s) { xmlFree(s); };
               ^
frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp:236:38: note: lambda closure types are non-literal types before C++17
constexpr auto xmlDeleter<xmlChar> = [](xmlChar *s) { xmlFree(s); };
                                     ^

Bug: http://b/110779387
Bug: http://b/111067277
Test: Builds with new compiler.
Change-Id: Ia7ed01bf0ff064f422be2cf6026cfe8329c404bf