Correctly handle passkey of zero

Previously, we treated 0 as "no passkey" / do not display passkey. Now
we don't.

I don't know how this handles the case where the remote device provides
0 and we auto-accept - can this be tested?

Test: manual
Tag: #stability
Bug: 241173545
Bug: 238653129
Change-Id: I992d1ae120b949eec19414b1cd08bdf40797706a
Merged-In: I992d1ae120b949eec19414b1cd08bdf40797706a
Ignore-AOSP-First: This change is already merged to AOSP(aosp/2184176)
(cherry picked from commit 73b914e56629ab3da3beda0deedda067849ed248)