Use system property for sniff idle time in HS

Notifications take longer time to enter sniff mode if HFC is connected.
Use bluetooth.bta_hs_sniff_delay_ms.config to replace the hardcoded
7000ms so we can adjust the sniff time

Bug: 263323082
Bug: 229316487
Test: Connect, enter sniff mode
Change-Id: I200ad8b402814115d0460c5f627aa3becf3c0d62
(cherry picked from commit a6f472f1eea71517188d0849897949f7bb6739d7)