MdeModulePkg/PrintDxe: Handle the deprecation of [A|U]ValueToString

MdeModulePkg/PrintDxe: Handle the deprecation of [A|U]ValueToString

To handle the deprecation of PrintLib APIs UnicodeValueToString and
AsciiValueToString by subsequent commits, the commit refines the logic for
the implemetation of the UnicodeValueToString and AsciiValueToString
services in EFI_PRINT2_PROTOCOL.

When the macro DISABLE_NEW_DEPRECATED_INTERFACES is defined (indicating
the deprecation of the PrintLib APIs), the above two services will ASSERT
and will return zero to reflect not being supported.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>