Power key behavior tweaks.

  - Added a new value for config_longPressOnPowerBehavior:
    "5" means "launch assist intent"
  - Added global settings for power key longpress and "very
    long press" (3.5 sec), allowing these functions to be
    tweaked at runtime by privileged code

Bug: 130415373
Test: adb shell settings put global power_long_press 5
      # ^ set longpress to assist
      adb shell settings put global power_very_long_press 1
      # ^ set vlongpress to globalactions (so you can still
      # power off the device)
Test: atest frameworks/base/core/tests/coretests
Change-Id: Ic611b2850dc98fddd7662d32b0a60ca1e8231efb