logd: Allow setDropped() to be called on already dropped messages

rprichard@ pointed out a bug where LogBufferElement::setDropped()
reallocates mMsg to the size of mMsgLen in the case where getTag() !=
0.  However, mMsgLen is in a union with mDroppedCount and
mDroppedCount is the value used when a message is already dropped.
Therefore, it's possible that logd uses the wrong value and allocates
much more memory than intended.

We do call setDropped() on elements that have already been dropped in
LogBufferElementLast::coalesce(), so this is not a superfluous issue.

To simplify this even more, this code puts an mTag in a union with
mMsg, such that if mDropped is true, there will never be an allocated
message; mTag will be directly referred to instead.  This also reduces
the number of allocations needed very slightly.

Test: logd/liblog/logcat unit tests.
Change-Id: Ia1bfba076439fe31c745a243283d41902bca45ac