Separate storing config for MakeVars from building boot image

Previously, the initialization of the dexpreoptBootJars fields used to
store information needed in its MakeVars method was interleaved with
the calls to buildBootImage(). In fact those fields were initialized
from the *bootImageConfig returned by buildBootImage(). However, the
method simply returned the *bootImageConfig value that was passed in.

Separating the initialization from the calls to buildBootImage() allows
the functionality to be moved to platform_bootclasspath separately.

Bug: 177892522
Test: m nothing
Change-Id: I8ba460f60be553516de5f186e14377bb0ec1b2cd