PhoneApp: Initialize ToneGenerator when needed

Cherry pick from AOSP.

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 not releasing the
instance between RAT changes.

Bug: 8609102
Change-Id: I2f2513624b66892018dc3191daf3e48c63f744a1
1 file changed