Switch boot image generation to use GetGlobalSoongConfig(ctx)

Previously, boot image generation used GetCachedGlobalSoongConfig(ctx)
in order to get access to the GlobalSoongConfig. That required that
some other part of the code had called GetGlobalSoongConfig(ctx) to
initialize the cached value. That was left over from when the boot
image generation was done in a singleton which could not call
GetGlobalSoongConfig(ctx) directly. That is no longer true.

This change switches the uses of GetCachedGlobalSoongConfig(ctx) to
GetGlobalSoongConfig(ctx) and removes the now unnecessary call to
GetGlobalSoongConfig(ctx) from outside the functions.

Bug: 192575099
Test: m nothing
Change-Id: I34b7b1526d072d8b09fda7caa96e381695888e16