Add the library absence check as a kati stage check.

Primarily for use with build_test during presubmit.

Moved the check to main.mk to avoid nonobvious dependency on
product_target_FILES from main.mk to Makefile.

Test: m systemimage
Test: build/soong/build_test.bash
Test: m out/target/product/taimen/system/lib/libdexfile.so; m systemimage;
  check that "find"-based check fails
Test: Add a system library to APEX_MODULE_LIBS; m systemimage;
  check that "product_target_FILES"-based check fails;
  env DISABLE_APEX_LIBS_ABSENCE_CHECK=true m systemimage;
  check that it works
Bug: 124293228
Change-Id: I7a93cc0ac4480404a3d56c75eb43ae499e8a3a05