ShellPkg DpDynamicCommand: Use new EfiLocateFirstAcpiTable()
    
    https://bugzilla.tianocore.org/show_bug.cgi?id=967
    Request to add a library function for GetAcpiTable() in order
    to get ACPI table using signature as input.
    
    After evaluation, we found there are many duplicated code to
    find ACPI table by signature in different modules.
    
    This patch updates DpDynamicCommand to use new
    EfiLocateFirstAcpiTable() and remove the duplicated code.
    
    Cc: Younas khan <pmdyounaskhan786@gmail.com>
    Cc: Michael D Kinney <michael.d.kinney@intel.com>
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Jiewen Yao <jiewen.yao@intel.com>
    Cc: Ruiyu Ni <ruiyu.ni@intel.com>
    Cc: Dandan Bi <dandan.bi@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Star Zeng <star.zeng@intel.com>
    Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>