Remove usage of ANDROID_BUILD_TOP

It will shortly become an error to read ANDROID_BUILD_TOP from make,
since it's not guaranteed to be available (it's only set in envsetup.sh,
which is not required to do a build).

Test: m out/target/product/generic/obj/PACKAGING/continuous_instrumentation_tests_coverage_intermediates/api_coverage.html
Change-Id: Ibfa171f8643c431cd3db5d668ac76e782a168636