ANDROID: usb: f_accessory: Wrap '_acc_dev' in get()/put() accessors

The '_acc_dev' global variable is a fancy use-after-free factory. Wrap
it in some get()/put() functions in preparation for introducing some
refcounting.

Bug: 173789633
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I4c839627648c209341a81efa0c001c8d71b878d4
Signed-off-by: Giuliano Procida <gprocida@google.com>