GATT: Expose opportunistic client API to Java

* Opportunistic GATT connection allows system service to subscribe to
  characteristic notifications without holding the BLE connection
* Exposing this flag to Java allows Android applications to use it

Bug: 63347806
Test: make, test on Android App
Change-Id: Ic470acd63bf45d1c33696abed1e93b30746fb7a2
Merged-In: Ic470acd63bf45d1c33696abed1e93b30746fb7a2
(cherry picked from commit 5120fedf20eb29fa65cef561c475d18d51197b25)