Add 'userspace-freezing' support to the capacity_capping test

Created by: credp

Running capacity_capping on a busy android system is an almost guaranteed failure, since it's actually quite reasonable to leave things running on very low OPPs if all the other cpus are already busy. However, if we freeze the userspace then we can be sure that the system is in a state where it should have the expected behaviour.

The test is not based on the executor class, so I took a few liberties rather than rewrite it using the new test classes. I reused the 'executor' label for the log entries so that the default logging.conf was able to display them. I took the implementation of freeze/thaw from executor, along with the task list.

Hope it's acceptable.