ShellPkg/alias: Fix bug to support upper-case alias

ShellPkg/alias: Fix bug to support upper-case alias

alias in UEFI Shell is case insensitive.
Old code saves the alias to variable storage without
converting the alias to lower-case, which results
upper case alias setting doesn't work.
The patch converts the alias to lower case before saving
to variable storage.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Tapan Shah <tapandshah@hpe.com>