add powerbtnd to simulate long press of power button.

add powerbtnd to simulate long press of power button.

On x86 PC, power button usually generates key press and release events
simultaneously. However, the android framework expects a long press
of power button to invoke the power off dialog. The daemon simulates a
long press of power button,

On poweroff.doubleclick=0, one click to power button invokes poweroff
dialog directly.

On poweroff.doubleclick=1, one click to power button suspends the system,
while double click in one second invokes poweroff dialog.

A target needs to add excluded-input-devices.xml to exclude
'Power Button' from processing by EventHub.