Bug 905107: Support RIL_REQUEST_SCREEN_STATE in reference RIL

With this commit, the reference RIL library supports the command
RIL_REQUEST_SCREEN_STATE. When received, the RIL daemon changes
the network-registration state by sending AT+CREG to the modem.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Merge remote-tracking branch 'b2gpriv/master' into b2g-jellybean