WirelessChargerDetector: Work around bad sensor timestamps.

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