Remove global state from apex modules

A global variant was used to store the global mapping between
modules and APEXes.  Replace it with storing pointers to APEX
contents inside each module so that they can query the contents
of any APEXes they belong to.

Bug: 146393795
Test: all Soong tests
Test: single line change to build.ninja host install dependency ordering
Test: no Android-${TARGET_PRODUCT}.mk, make_vars-${TARGET_PRODUCT}.mk or late-${TARGET_PRODUCT}.mk
Change-Id: Id2d7b73ea27f8c3b41d30820bdd86b65c539bfa4