Use btif_merge_existing_uuids when reporting GATT results

When we receive service discovery results, we immediately filter them.
There is no need to filter them again when looking them up with
btif_merge_existing_uuids. When the filtering is removed from
btif_merge_existing_uuids, it can be used to merge existing records in
GATT results processing.

Bug: None
Test: mma -j32
Flag: exempt, minor code improvement
Change-Id: I4baf76c16a47f2a1ad3d100542f115e82446ae09