logd: isMonotonic improvements

Use 1972 as a right delineation. Otherwise use half way point
between the monotonic and realtime. Treat correction factor as
unsigned, ensure that any wrapping to a negative value is
dropped or set to EPOCH. Acknowledge that we can get a more
accurate time track by acquiring the time rather than relying on
healthd timestamp.

Bug: 26331432
Change-Id: I09075fca58676a30cf7d87baf2d4b0f53795abaa