Fix race condition when doing GATT discovery

Right now if discovery to multiple GATT clients is happening
simultaneously, onSearchComplete will be run only for device that
finishes discovery last, while it should be run for each device.

mSearchQueue is one for all discovery sessions. Instead of checking if
it's empty, we must check if there are no elements for given connId.

Bug: 26038939
Change-Id: I9417cd7be8cab4b808ce7f045861e1adc2055629