Enable HID Device Profile Service (2/2)

Add Profile Proxy for HID Device Profile.

* Add a helper method doUnbind() to deal with unbinding to Bluetooth HID
Service.

* Fix docstrings.

Bug: 63384609
Test: SL4A Bluetooth HID test
Change-Id: I168c21c3b010baac9889c556635b914c0ba3f267