Add a new UEventObserver subclass to broadcast an Intent whe USB state changes.

We now broadcast Usb.ACTION_USB_CONNECTED and Usb.ACTION_USB_DISCONNECTED
when USB is connected or disconnected.
The ACTION_USB_CONNECTED extras indicate the enabled/disabled state of
all USB functions.

Change-Id: I11495d039429dbe22bd738067296e39ae415befa
Signed-off-by: Mike Lockwood <lockwood@android.com>