GATT: Expose opportunistic client API to Java

* Allow Java based programs to create an opportunistic GATT client
* Such client does not hold a GATT connection. It automatically
  disconnects when no other GATT connections are active for the
  remote device.

Bug: 63347806
Test: make
Change-Id: I7fe789b38a25c65b5bc4f1591e406edb7742bd5b