In our tests, it was possible for a timeout to fire after |clearTimeout|
was called. This CL makes sure that cannot happen by adding an "alive"
flag to each timeout.
To ensure that the change was as well defined as possible, this CL
changes how |shaka.util.Timer| is implemented and updates the API to
better communicate what is happening internally.
Change-Id: I57e3899046a762bff3293b9822a7e8f7ac804042