Fade out inattentive sleep warning overlay

After long user inactivity, a warning overlay is shown that the device
will go to sleep soon unless a button is pressed.

Currently, the overlay is hidden immediately on user activity.
This causes the overlay to be dismissed on an ACTION_DOWN KeyEvent,
and the following ACTION_UP KeyEvent to be delivered to the app
underneath the overlay.
We want to consume the ACTION_UP KeyEvent, as eg. an ACTION_UP KeyEvent
for the back button causes YouTube to quit out of a currently playing
video (even if the KeyEvent is cancelled).

With this change, the overlay fades out, and during the animation
continues to receive KeyEvents.
For quick button presses, this prevents the app from seeing the
ACTION_UP KeyEvent.
For long button presses, the app continues to see key repeats
and ACTION_UP events.

Bug: 137633812
Test: 1. Set timeout lower:
         `adb shell settings put secure attentive_timeout 35000`
      2. Play YouTube video
      3. Wait for overlay to show
      4. Press back
      5. Verify: Video keeps playing, overlay disappears
Change-Id: Ie6eab2b381f3ba9638b231cfeab3b90ad349b6e8