NetworkPkg/DnsDxe: Fix zero StationIp configuration failure of DNSv6

NetworkPkg/DnsDxe: Fix zero StationIp configuration failure of DNSv6

According UEFI Spec, set to zero StationIp means to let the underlying
IPv6 driver choose a source address. But currently, DNSv6 always return
EFI_NO_MAPPING. The issue is caused by below bugs in DnsDxe:
* Incorrect TPL(TPL_CALLBACK) usage during UDP configuration.
* Failed to create the timer used to get IPv6 mapping
* Doesn't check the Ip6Mode.IsStarted flag.

Cc: Zhang Lubo <lubo.zhang@intel.com>
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: Zhang Lubo <lubo.zhang@intel.com>