logd: remove LogFindWorst

LogStatistics relies on LogBuffer's lock for thread safety, but that
will be cleaned up in future CLs.  It won't be possible to return a
'LogFindWorst' object that references internal LogStatistics pointers
in a thread safe way, so we remove this and provide a more simple
interface.

This also removes unnecessary allocations; std::array of 2 or 32
entries is small enough to allocate on the stack.

Test: logging unit tests
Change-Id: I38bfcba5b08c640ffd3af5c078bc716688f6edf8