Power + Volume up key chord can now be configured.

Device default value set in config_keyChordPowerVolumeUp in
config.xml; can be overridden for all users with
Settings.Global.KEY_CHORD_POWER_VOLUME_UP.

Value may be one of:

    0 - no-op
    1 - vibrate mode (current AOSP default behavior)
    2 - launch assistant

Bug: 179673796
Test: adb shell settings put global key_chord_volume_up <0, 1, or 2>
      adb shell dumpsys window | grep mPowerVolUpBehavior
      adb shell input keycombination POWER VOLUME_UP
Change-Id: I0e03155bdbe61d9fdd6838fe2c860749cf360907