ArmPkg/PlatformBootManagerLib: fall back to the UiApp on boot failure

ArmPkg/PlatformBootManagerLib: fall back to the UiApp on boot failure

As a last resort, drop into the UiApp application when no active boot
options could be started. Doing so will connect all devices, and so
it will allow the user to enter the Boot Manager submenu and pick a
network or removable disk option.

Note that this only occurs if even the default removable filepath
could not be booted (e.g., \EFI\BOOT\BOOTAA64.EFI on AArch64)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>