HeadsetPhoneState: Synchronize constructor with phone state listening functions.

Bug: 241871296

The bug is that mSignalStrengthUpdateRequest is null when bt calls mTelephonyManager.clearSignalStrengthUpdateRequest(mSignalStrengthUpdateRequest).

mSignalStrengthUpdateRequest is created at constructor. So the only explanation is that there is a race condition between the constructor and stopListenForPhoneState() who calls clearSignalStrengthUpdateRequest();

Test: atest BluetoothInstrumentationTests
Change-Id: I0d456a4a7981e1befe291d2045b926a4da4b6bf5