Revert "logd: drop mSequence from LogBufferElement"

This reverts commit 5a34d6ea.

There is a long standing bug that logd will leak memory during its
prune process if the time on the device changes significantly forwards
then backwards.  This is due to using the timestamp of each log
message to determine what log messages are yet to be processed by a
reader thread.

Various attempts have been made to rectify this, but the only solution
that safely fixes this issue is to go back to using sequence numbers
on the log messages.

Bug: 64675203
Bug: 77971811
Bug: 149340579
Bug: 150923384
Test: logcat output looks sane
Change-Id: Ibce79cf184eb29a4914f3e42a8cb2868d04dc165