Fix race condition during simultaneous SDP

Right now sdp_conn_id and p_sdp_db are stored in static global
bta_gattc_cb between call to bta_gattc_sdp_service_disc and
bta_gattc_sdp_callback. If multiple instances of SDP discovery
are running simultaneously, they override this field, and free
same memory multiple times. This patch fixes that by making sure
sdp_conn_id and p_sdp_db are unique for each SDP discovery.

Bug: 25801255
Change-Id: I8ec52229e906e6b8748db7504f77e1f4d7006fbe