Implement alternate power button behavior for dreams.

This change implements a power button behavior that prioritizes entering
a dream on press, rather than turning off the screen.

Bug: 245961904
Test: Manually as follows:

Make sure config_shortPressOnPowerBehavior is set to
SHORT_PRESS_POWER_DREAM_OR_SLEEP.

1. Set screensaver to dock then dock device. Make sure tapping power
   button when screen on goes to dream. Make sure tapping power button
   when screen off goes to dream. Make sure tapping power button while
   dreaming turns display off.
2. Set screensaver to charge then plug in device. Make sure tapping
   power button when screen on goes to dream. Make sure tapping power
   button when screen off goes to dream. Make sure tapping power button
   while dreaming turns display off.
3. Set screensaver to charge or dock and perform the same tests as
   above.
4. Set screensaver to dock and then undock device. Make sure tapping
   power button does not start dreaming.

Note that the device may charge while docked, so setting screensaver to
charge may act the same when docked or charging.

Change-Id: I9b6859b4933839841b76b832f275dcc14313f192