Fix VR surface attributes.

VR surface attributes had two issues that prevented the full extent
of their use:
1. The older clang version missed that templated copy constructor
   and assignment operators do not override the default ones in
   the Variant class. This caused issues with certain types when
   copy constructing / assigning from another Variant of the same
   type. This was noticed by running tests with a newer version of
   clang which provided warnings.
2. C++ rules about implicit conversion to bool from types that
   decay to pointers causes subtle issues with Variants that have
   bool elements.

   For example this assignment compiles but produces the wrong
   result:

   const int array[3] = { 1, 2, 3};
   Variant<int, bool, std::array<int, 3>> variant = array;
   EXPECT_FALSE(variant.is<bool>()); // Actually true.

   Here the programmer might accidentally think that the std::array
   element of the variant can be assigned from the regular array.
   This doesn't work, but instead the compiler decays the array to
   a pointer and assigns the bool element to true.

The first issue is addressed by defining copy/move constructors /
assignment operators on Variant and deleting the default ones from
the internal Union type for extra safety.

The second issue is addressed by making a more restrictive version
of the std::is_constructible trait that rejects bool construction
from types that decay to pointers. Once this was put in place the
erroneous use cases no longer compiled and is fixed as part of
this CL.

Tests are updated to verify the fixes to these issues.

Bug: 62557221
Test: pdx_tests and dvr_api-test passes.
Change-Id: Id4647e54e0a7b1753217fe7fe351462fe5bcfd83