It turns out that on some devices the sensor event timestamp
may be non-monotonic. This may cause the detector to hold
a wakelock indefinitely if the time jumps backwards. These
timestamps are not well tested in general so there may be
other artifacts as well. Use elapsed realtime instead.
Bug: 9926451
Change-Id: Idb0b316e22b23aac86837bd25b953daf49f9b758
20e2875343