PBAP: Improvisation in start time of BluetoothPbapService

This change is an improvisation to reduce time to start PBAP
Service by moving task of getting local phone details from
start() method running in main Bluetooth context to PbapHandler
Thread. Getting phone details from telephony service was adding
around 20ms in starting PBAP Profile and thus increasing time
for Bluetooth startup.

Test: Calculate timetaken to turn on BT and observed reduction of
20 ms.

Bug: 118604915
Change-Id: I76c9e4846e2eb6048125ea6e942860c55705d113