Fix timebase for sendMessageAtTime in LowPowerStandbyController

Handler#sendMessageAtTime takes a timestamp in the uptimeMillis
timebase, but a timestamp in the elapsedRealtime timebase was passed in.
As elapsedRealtime continues to tick while in deep sleep, uptimeMillis
is behind elapsedRealtime after deep sleep and messages were delayed.

Test: atest LowPowerStandbyControllerTest
Test: manual, after waking TV from deep sleep
Bug: 296481637
Change-Id: Ia69ad148151980821aaa18c653c408c65581e7ed