GD Hack: Start service discovery on bonded

We haven't unified the callback path from BTM or GD shim to BTA/BTIF
yet.  We have to start service discovery specifically for GD at the
moment.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Test: connect to a speaker
Change-Id: I958c99b6943be36469096e8039ff95fda0026a0b