MdeModulePkg/Ip4Dxe: Trigger Ip4Config2 to retrieve the default address.

MdeModulePkg/Ip4Dxe: Trigger Ip4Config2 to retrieve the default address.

According the UEFI spec 2.7 A:
In section 28.3.2 for the IpConfigData.UseDefaultAddress, "While set to
TRUE, Configure() will trigger the EFI_IP4_CONFIG2_PROTOCOL to retrieve
the default IPv4 address if it is not available yet."
In section 28.5 for the Ip4Config2PolicyDhcp, "...All of these configurations
are retrieved from DHCP server or other auto-configuration mechanism."

This patch is to align with the above description. When the default IPv4
address is not available and IpConfigData.UseDefaultAddress is set to TRUE,
Ip4Config2 protocol will be called to retrieve the default address by setting
the policy to Ip4Config2PolicyDhcp.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>