Fix race in HeadsetPhoneState

By having variable always initialized and final, they cannot be null at
any point and we don't have the risk of a race condition when creating
the object (post would run before the end of the constructor)

Bug: 396979840
Fix: 396979840
Test: m Bluetooth
Test: atest BluetoothJavaUnitTests
Flag: Exempt, mechanical no-op refactor
Change-Id: I7e755a2fdaea94f82c8d3a381aecc6dcdc9fcb6d