Fix atest error of no attribute 'coverage'

This is because the arg of 'coverage' be removed but still be used in
the atest_main.py. Fix it by change them to 'experimental_coverage'.

Test: m atest && atest-dev HelloWorldHostTest
Bug: 265373975
Change-Id: I14495b28c73d0ceb11d3e59e168bd9623baa82af
1 file changed