Assign pairing code to bta_dm_cb before device name resolution

Device name resolution, BTM_ReadRemoteDeviceName(), will cause
bta_dm_sp_cback() to exit early and bta_dm_pinname_cback() callback
will be invoked after the resolution, which continues the pairing
with pairing code from bta_dm_cb.num_val. Hence, bta_dm_cb.num_val
needs to be assigned before BTM_ReadRemoteDeviceName() is called.

Test: manual
Bug: 31381715
Change-Id: I61f06a9d878dd72154d6621eb094dcea5f701cbc
(cherry picked from commit 23790f91)