ANDROID: usb: f_accessory: Add refcounting to global 'acc_dev'

Add refcounting to track the lifetime of the global 'acc_dev' structure,
as the underlying function directories can be removed while references
still exist to the dev node.

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I248408e890d01167706c329146d63b64a6456df6
Signed-off-by: Giuliano Procida <gprocida@google.com>