ThermalManagerService: fix different system clock comparison

ThermalManagerService tried to compare
SystemClock.elapsedRealtime() and System.currentTimeMillis().
It may lead to more unnecessary update tasks.

Ref: https://developer.android.com/reference/android/os/SystemClock
Add testcase testTemperatureWatcherGetForecastUpdate

Bug: 203028173
Test: atest ThermalManagerServiceTest
Change-Id: Ic0c579cb124fd8d86bff9f6d9e21e33ca9d9f0ff