logd: Drop the LogStatistics dependency on LogBufferElement

Other log buffers may not use LogBufferElement, so we should decouple
the two classes.  This uses an intermediate LogStatisticsElement
structs instead of passing a large number of parameters to each
function.

This additionally moves IsBinary() and the GetTag() functions out into
LogUtils.h since they can be used generically by other users.

Test: logging unit tests
Change-Id: I71f53257342c067bcccd5aa00bae47f714cd7c66