core: Add a standalone charger UI binary

This can be launched from an 'on charger' section in init, which
is processed if androidboot.mode=charger is supplied on kernel
command line.

This is a standalone binary that has a simple "user interface"
and allows the standard production kernel to run while charging.
This removes the burden from the bootloader to get low-power mode
working.

Currently, the device will "power-on" (i.e. reboot) into normal
mode if the power key is pressed for X seconds, and will also
power down the device if the usb/ac cable is removed for Y seconds.

Change-Id: I780594b99211cb09de7f8519a69c5962eb91348a
Signed-off-by: Dima Zavin <dima@android.com>