BTA DM: Clean up name read loop.

In bta_dm_discover_name, when we pass through whole method, and there is
no need to read remote name, we end up sending BTA_DM_DISCOVERY_RESULT_EVT.

This event is now jointly used by service discovery, and finding
devices. Because of this mix up, handler of this event is sending
service discovery and device search callbacks together.

To clean up this mess, use BTA_DM_DISCOVERY_RESULT_EVT only when service
discovery result is present.

When name discovery is finished, call next device name discovery
directly.

Please note that bta_dm_search_cb.wait_disc is never set when reading
device name, so there is no need to keep the checks and timer from bta_dm_search_result

Bug: 330675788
Flag: exempt, infrastructure refactor that can't be easily isolated
Test: mma -j32
Change-Id: I416af873d753c37d8e4349a731aef8076a177170