Add acov for coverage gathering/reporting.

acov is a tool for gathering coverage information from a device and
generating a report from that information. To use:

1. sudo apt-get install lcov
2. Build application/library with coverage information.
3. Push the new binaries to the device.
4. Run tests with the additional environment variables:
    * GCOV_PREFIX=/data/local/tmp/gcov
    * GCOV_PREFIX_STRIP=`echo $(ANDROID_BUILD_TOP) | grep -o / | wc -l`
5. Run `acov`.

acov will pull all coverage information from the device, push it to the
right directories, run lcov, and display the coverage report (currently
by opening it in your browser).

Bug: 10134489
Change-Id: I604c8ec2e9fa5d23ee9c279b515ff6cfc497dfa7
Merge "Add acov for coverage gathering/reporting."