logd: getTag() functional for chatty entries

getTag() becomes invalid when entry is dropped because mMsg
disappears to save space; but the per-tag spam filter depends on it
still being valid.  Conserve space in LogBufferElement by optimizing
the size of the fields, then add a new mTag field that is set in the
object constructor.  Add an isBinary() method.

SideEffects: save 12 bytes/log message overhead on 64-bit.
Test: define DEBUG_CHECK_FOR_STALE_ENTRIES and look for stale entries
Bug: 32247044
Change-Id: Iaa5f416718a92c9e0e6ffd56bd5260d8b908d5c0