Don't read HearingAid properties if already have properties value.

When read HearingAid properties, don't read it if already have the
HearingAid properties in config file. Will use these values in config
file since these values should never change.

Bug: 128464408
Test: 1.Forget/Repair HearingAid device from Bluetooth UI.
2.Disconnect/Reconnect HearingAid device from Bluetooth UI.
3.HearingAid device power off/on, check them can reconnect.

Change-Id: Id4c30947c75f0615514075ec36c50afb58fc3f14