gatt: properly call connection callback when connection is already up

the callback might be called before this func returnes (talk
to your scheduler about that). This leaves you with a conundrum:
How will you know the callback is for you when you do not yet
know thw connection id. Caller solves this themselves. eg: take
a mutex in callback, and also take it before calling this, do
not release it till this func returns.

BUG=b:74813838,crbug:905047
TEST=test app unbroken
Change-Id: Ie4918e525057e228695b2f2bfa1d9fb96c180899
1 file changed