jni: introduce ScopedBtAddr

ScopedBtAddr handles marshalling bt_bdaddr_t into jbyteArrays so we
don't have to.  It also complains when it can't allocate things and
deletes the local reference when it goes out of scope.

Test: compiles, sanity tests with various profiles

Change-Id: I78a6364cc5cac0e8664d2d04388a6e2ac4c7a58f