MdeModulePkg TerminalDxe: Execute key notify func at TPL_CALLBACK

MdeModulePkg TerminalDxe: Execute key notify func at TPL_CALLBACK

Current implementation executes key notify function in TimerHandler
at TPL_NOTIFY. The code change is to make key notify function
executed at TPL_CALLBACK to reduce the time occupied at TPL_NOTIFY.

The code will signal KeyNotify process event if the key pressed
matches any key registered and insert the KeyData to the EFI Key
queue for notify, then the KeyNotify process handler will invoke
key notify functions at TPL_CALLBACK.

Cc: Ruiyu Ni <Ruiyu.ni@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <Ruiyu.ni@intel.com>