Bluetooth HID Device: format code, fix docstring, hide unplug()

* Reformat code with google-java-format
* Add the note that When an application is registered, the HID Host service
will be disabled until it is unregistered
* Fix the links in docstring
* Add @hide to unplug(); this is not a public API

Bug: 63384609
Test: make
Change-Id: I5dfcaab58b02c19e5745461c16602064a0ad8b83