rilmodem: Changes to support mtk modem

* MTK modem specific behaviour is addressed
* spn-table plugin support is added to network-registration
* Change in rilmodem sim interface
* Fix not checking properly null pointer in gprs atom
* gril changes are addressed
* Add file with list of rilmodem modem types
mtkmodem: New mtk modem driver

This driver provides support for MediaTek modems. Most of the
functionality is already provided by the rilmodem driver: this driver
provides support for specific MTK parcels and voicecall and gprs atoms.
These atoms are separated from those in rilmodem as they use specific
MediaTek requests and events. This will allow us to move the driver to
a dynamically loaded plugin if desired.
include: New interface for spn-table type plugins

This new plugin type allows adding plugins that can be used to search
for operator names if the modem is not able to find it for a given
MCC/MNC pair.
src: Add support for spn-table type plugins
plugins: Add android-spn-table plugin

This plugin looks for the SPN in the spn-conf.xml Android file.
build: Add plugins for MTK modem support

* Add mtk, androidspntable, and mtkmodem plugins
* Add support for spn-table type plugins