EmbeddedPkg/VirtualKeyboard: Avoid notification called more than once

EmbeddedPkg/VirtualKeyboard: Avoid notification called more than once

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=996

Issue:
In current code logic, when a key is pressed, it will search
the whole NotifyList to find whether a notification has been
registered with the keystroke. if yes, it will en-queue the
key for notification execution later. And now if different
notification functions have been registered with the same key,
then the key will be en-queued more than once. Then it will
cause the notification executed more than once.

This patch is to enhance the code logic to fix this issue.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>