Count and log the number of consecutive rapid power button presses.

Here, "rapid" is defined as 500ms.
The histogram will allow us to measure the basal rate at which multiple
rapid power button presses occur, in order to estimate the false positive
rate a consecutive rapid power button sequence would generate.

Test: Unit tests for all values logged.
Also, manual testing of existing power button feature and inspection of
logcat output.
Manual test cases executed:
1. Screen off, power button tap turns screen on.
2. Screen on, power button tap turns screen off.
3. Screen off, power button double tap triggers camera.
4. Screen on, keyguard visible, power button double tap triggers camera.
5. Screen on, bouncer visible, power button double tap triggers camera.
6. Screen on, launcher visible, power button double tap triggers camera.
7. Screen off, power button long press turns screen on.
8. Screen on, keyguard visible, power button long press shows power off
   / restart menu.
9. Screen on, bouncer visible, power button long press shows power off
   / restart menu.
10. Screen on, launcher visible, power button long press shows power off
    / restart menu.

Change-Id: I3d96f55ab7c541338106b43bc48e6a93a0444891