Update BluetoothPbap class description to indicate how to create an

instance of the class, add intdef to return value of
BluetoothPbap#getConnectionState, and move all SystemApis to require
the BLUETOOTH_PRIVILEGED permission

Bug: 148966894
Test: Manual
Change-Id: I4a798b0a16ab839f2047fc58f21c420cc99b6db6