logd: simplify Clear() + Prune() logic

Clear() and Prune() return a boolean indicating whether or not their
operations failed because the log buffer was 'busy'.  This means that
they return false upon success and true upon failure, which is not
intuitive.

This change inverts their return value to simply be true if they were
successful or false otherwise.  It also simplifies the return value of
ChattyLogBuffer::Prune() to true if the requested number of rows have
been pruned or if all rows in the log buffer have been pruned, and
otherwise return false.

Test: logging unit tests
Test: clearing works even under logging pressure
Change-Id: I346bb945496ef62bf8e973298f81c5163f49bc57