input: sensors: send boot time alone with sensor events

New Android CDD suggest that sensor event should synchronize with
SystemClock.elapsedRealtimeNano() clock. Send boot time alone with
sensor events to represent the time the event happened as many as
possible and synchronize with that clock.

Change-Id: I3e6d111d0f1571786c1075e01242de7f1e121ce1
Signed-off-by: Bingzhe Cai <bingzhec@codeaurora.org>