ShellPkg: ShellFileHandleReadLine must return UCS2 lines.

ShellPkg: ShellFileHandleReadLine must return UCS2 lines.

An earlier change had this function returning the type of lines that were in
the file being read (ASCII or UCS2). The way it is used, UCS2 output is
expected, even when the file being read is ASCII. This change restores that
behavior and documents it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jim Dailey <Jim_Dailey@Dell.com>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>