telephony: SignalStrengh: Don't calculate a LTE snr if the srq is invalid

If the previous values are invalid and signal quality is -1, chances are there's
no LTE signal at all, so don't calculate the SNR and mark it invalid as well.

This fixes the strength indicator in GSM devices where it was being skewed
by the ghost LTE value

Change-Id: I070573401c141aa99941a332ee256246697abb90
Framework: Switch to the new 4.2 handling of public symbols

With 4.2, non-api public symbols are now kept in the symbols.xml
file and no longer in the public.xml file.  This commit moves the
CyanogenMod required symbols to the proper file.

Thanks to PawitP for pointing it out

Change-Id: Ied3ea11c91eb8e8947a18b11b6d49f3a09bcbee5