Fix to read Tethering APN data terminated by empty parameters correctly

If Tethering APN data is terminated by empty parameters(",,,"), the
tethering APN data is not read correctly from configuration file.

Because second argument of split() is not set in ApnSetting.java, these
empty parameters are ignored and an error occurs during parameter length
check later.

To fix this, second argument of split() should be set as "-1".

Bug: 112876857
Test: manual - Checked that Tethering APN data could be read correctly.
Change-Id: I18be0131287dc927334d51522f1c816da83d4add