Settings: Convert notifications to use NotificationChannel (2/2)

Test Setup:
===========
HW DUT: Pixel O
Remote: MecApp (PBAP Client, MAP Client, SAP Client)

Steps:
=====
1. Enable BT on DUT and bond with Remote.
2. Connect from MecApp for PBAP, MAP, SAP and observe if
   connection goes through fine.

Reproducibility:
===============
5/5

Observation:
============
MAP, PBAP and SAP cannot be connected.

Root cause:
In Android O, notifications are hidden by default.
This must be changed to use notification channels so that the
priority of the notifications can be updated.

Test: PBAP/MAP/SAP connection works fine with MecApp.

Bug: 38331825
Change-Id: I51de0ea303037bf88773352d99f092673acda2e3
(cherry picked from commit bd9f532013f0397879101f68f21ac8841fec344b)