ArmPkg/PlatformBootManagerLib: refer to Shell FILE_GUID directly

Instead of indirecting the reference to the Shell binary via a PCD
that is defined in IntelFrameworkModulePkg, and which invariably
gets set to the same value by all users of this library, refer to
the UEFI Shell application by its declared symbolic GUID.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>