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: If4683b2281d743dacdece6b08db49ec539af3b18
Merged-In: If4683b2281d743dacdece6b08db49ec539af3b18
(cherry picked from commit 56f19a1f1188aced72103e7971603267519bdbed)
1 file changed