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, run battery service
Change-Id: Ib9333817d7f17a1fa3ddacfa51c680890bac19ec