PBAP: Convert notifications to use NotificationChannel

Test Setup:
===========
HW DUT: Pixel O
Remote1: MecApp PBAP Client

Steps:
=====
1. Enable BT on DUT and bond with Remote.
2. Connect from MecApp PBAP Client Tab and observe.(Request
   Authentication checkbox enabled in MecApp).

Expected Result:
================
Device should connect successfully.

Actual Result:
===========
In Step2, "The remote PBAP server did not accept the connect request error
  popup is displayed."

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

Log trace:
Line 6855: 05-08 14:04:23.453 10051 11015 D BluetoothPbapService:
Pbap state 0 -> 2, result = 1
Line 6868: 05-08 14:04:23.465  1615  1751 E NotificationService:
Cannot post notification
without channel ID when targeting O  - notification=Notification
(pri=0 contentView=null vibrate=null
sound=default tick defaults=0x1 flags=0x18 color=0xff607d8b vis=PRIVATE)
Line 6947: 05-08 14:04:33.471 10051 10051 W BluetoothPbapService:
Release Wake Lock
Line 7155: 05-08 14:04:53.491 10051 11492 D BluetoothPbapObexServer:
onClose(): msg MSG_SERVERSESSION_CLOSE sent out.
Line 7172: 05-08 14:04:53.509 10051 10051 D BluetoothPbapService:
Pbap state 2 -> 0, result = 1

Root Cause:
In Android O, Bluetooth 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 connection works fine when remote sends authentication
challenge.

Bug: 38149308
Change-Id: Ifff0a81912f1bf11edc73279a9a127b791188d02