:module syntax is supported for APEX even when TARGET_FLATTEN_APEX

Build rules for both flattened and non-flattend APEXes are created
regardless of TARGET_FLATTEN_APEX. The selection is made in AndroidMk.
This allows other module to reference an APEX via :module syntax
irrespective of TARGET_FLATTEN_APEX.

Bug: 123780484
Test: TARGET_FLATTEN_APEX=true m out/soong/.intermediates/art/build/apex/art-check-debug-apex-gen/gen/art-check-debug-apex-gen.dummy
with aosp/891696 applied
Change-Id: Ia49415ec3d18cfc5081461be76900c73ea803dca