Crespo4G: Voicemail: Add 'config_telephony_...' for Sprint

'*86' which is set now as a default at CDMAPhone.java,
is default voicemail number for Verizon.
For Sprint, we use user's own number for voicemail.

So we add codes in CDMAPhone.java to use
'config_telephony_use_own_number_for_voicemail',
and use config.xml to set this value as false.
Then we overlay Sprint's own config.xml file to
override 'config_telephony_use_own_number_for_voicemail' as a true.

Change-Id: I110914bdfa9a79aaba89d3b80edbcf044e9aabee