Floss: Auto generate pin code for keyboard-like devices

Following convention in Android and BlueZ, for keyboard-like devices,
hosts automatically generate a random pin code and display in the UI.
For other devices, hosts ask users to input a pin code.

Bug: 277980471
Test: run GAP/SEC/SEM/BI-05-C and check btclient receives pin code
request/display

Change-Id: I2357da33da3142131013f39c62cdd783ad004587