Move coverage builds prebuilt special case to make

Having this in make has the following advantages
- allows this to be used in other places in make that are sensitive to
  prebuilt selection, e.g. apex boot jars that are present in source but
  not in prebuilt apexes
- collocates the various prebuilt special edge cases

Test: presubmits
Test: in internal, lunch cf_x86_64_phone-next-userdebug
Test: EMMA_INSTRUMENT_FRAMEWORK=true get_build_var PRODUCT_BUILD_IGNORE_APEX_CONTRIBUTION_CONTENTS
true

Bug: 308188056
Change-Id: I3e81b348e9f1e72e6d120a881d37356c413b005b