liblog: logd: support logd.timestamp = monotonic

(cherry pick from commit b6bee331)

if ro.logd.timestamp or persist.logd.timestamp are set to the value
monotonic then liblog writer, liblog printing and logd all switch to
recording/printing monotonic time rather than realtime. If reinit
detects a change for presist.logd.timestamp, correct the older entry
timestamps in place.

ToDo: A corner case condition where new log entries in monotonic time
      occur before logd reinit detects persist.logd.timestamp, there
      will be a few out-of-order entries, but with accurate
      timestamps. This problem does not happen for ro.logd.timestamp
      as it is set before logd starts.

NB: This offers a nano second time accuracy on all log entries
    that may be more suitable for merging with other system
    activities, such as systrace, that also use monotonic time. This
    feature is for debugging.

Bug: 23668800
Change-Id: Iee6dab7140061b1a6627254921411f61b01aa5c2