floss: Support more pairing methods in OnPairing

Fix pairing events and pairing answer streaming.
In mmi2grpc, mutiple OnPairing streaming are allowed at the
same time. Pairing events will be streamed in a broadcast manner to
all gRPC client calls.

Add legacy pairing methods for pairing events. Add `passkey entry`
and `PIN code entry` pairing answers.

Bug: 312653014
Tag: #floss
Test: m Bluetooth && pts-bot GAP
Flag: EXEMPT floss only changes
Change-Id: I1ebf72e468e5bed3359b5bb0e3e78167e720a68e