Add `acov --clean` and `acov --prep`.

The --clean flag clears ANDROID_PRODUCT_OUT and ANDROID_HOST_OUT of
all coverage files. If a source file that had a coverage notes file
(a .gcno) generated for it is renamed or removed, the build system
will not clean the old file, which means the coverage tool will still
try to find coverage data.

The --prep flag clears just coverage data files. This should be done
before any coverage run, as you don't want old coverage data affecting
your results.

Change-Id: Id68395d19369eba4589daff90461a10cf7aef66d
1 file changed