uinput: fix timestamps for JSON-style recordings

The previous changes to support specifying timestamps for evemu
recordings inadvertently made injections from JSON-style recordings have
the timestamps at which the injection was scheduled, not when it
happened, causing many events to be injected with very close-together
timestamps. This broke drawing in the tests for some inking libraries,
such as Keep's.

Bug: 330844071
Test: $ atest 'PlatformScenarioTests:android.platform.test.scenario.sysui.stylus.StylusInkingTest#writeText_appearsInShowcaseApp'
      (with the @Ignore in StylusInkingTest.kt removed)
Test: replay the recording from b/330844071#comment30, check the curve
      is drawn correctly
Test: replay an evemu recording and check that the timestamps shown by
      `getevent -lt` are still correct
Change-Id: I5342a79d4f9b53875b9918443bf1f5fd16cd205e