BTA discovery/search: clean up logic for picking transport.

For device search , we should never check the control block content, and
rely on information we found about device.

For service discovery, it's enough to just pass transport, no need to
store it in control block.

Bug: 330675788
Bug: 335732980
Flag: com.android.bluetooth.flags.separate_service_and_device_discovery
Test: mma -j32
Change-Id: I1442857de0c0cfdd0edeb9f34fe7efe064001bd8