Add setting to go to sleep after long user inactivity

The European Commision prescribes an auto-standby feature for TV panels:
After some hours of inactivity, the device has to go from on-mode to
standby-mode or off-mode, or another condition not exceeding the
applicable requirements for standby-mode or for off-mode.

After a long time of no user activity the device should go to sleep,
even if wakelocks are held (eg. during video playback).

Test: 1. Set attentive timeout low, to 35s:
         `adb shell settings put secure attentive_timeout 35000`
      2. Play a YouTube video
      3. Observe warning dialog appearing after 5s
      4. Verify: Clicking a remote button or changing the setting higher hides
         the warning. Remote button press is consumed.
      5. Verify: After 35s of not pressing a button the device goes to sleep
      6. Verify: If "Stay awake" developer option is enabled, then
         warning is not displayed and device does not go to sleep after 35s
      7. Verify: No warning or sleep if setting is set to -1
Test: `atest frameworks/base/services/tests/servicestests/src/com/android/server/power/PowerManagerServiceTest.java`
Bug: 137633812
Change-Id: I551b6cffc336437fb1c5a00b4102f68ae0e003e9