Add USB port manager.

Add some new internal APIs to enumerate USB Type C ports, query their
status, determine whether they support changing power or data roles,
and doing so.  The API also adds a new ACTION_USB_PORT_CHANGED broadcast
for port state changes.

The implementation includes a mechanism for simulating the behavior
of the USB stack.  See 'adb shell dumpsys usb -h' for details.

Note that the underlying kernel driver interface is still subject
to change but its behavior has been encapsulated as much as possible.

Bug: 21615151
Change-Id: I0c853ae179248a4550b3e60d02a7a7e65e4546b2