Add millis suffix to BackEvent#getFrameTime API

Bug: 373656418
Test: OnBackInvokedCallbackGestureTest
Test: WindowOnBackInvokedDispatcherTest
Flag: com.android.window.flags.predictive_back_timestamp_api
Change-Id: I143a343ad0eedd63f40533c8adecb81be3e06066
8 files changed