PhoneApp: Initialize ToneGenerator when needed

Whenever there is a radio tech change, mSignalInfoToneGenerator
is released and never created. This leads to phone app crash later
when it is accessed for toneplay. This is fixed by creating the
instance (if null) before calling the APIs.

Bug: 8609102
Change-Id: Id9ebe6af5c1f92520ee5c2ca6efc4db335eb01f7