[floss] Add default RSSI sampling period

Add same default RSSI sampling period as BlueZ if none is provided
by the client.

BUG=b:328432018
TEST=unit tests

Change-Id: Ifcf7d253a857d8b79495200f10e7edaf1b26d7ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5362495
Commit-Queue: Michael Sun <michaelfsun@google.com>
Reviewed-by: Michael Sun <michaelfsun@google.com>
Auto-Submit: Katherine Lai <laikatherine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271168}