Implemented service isTelephonyReady API method

Without having the isTelephonyReady method available luna-systemui will continue to poll
our service until it gets a response which never happens and will cause the WebAppMgr
process to consume about 100% of the CPU cycles.

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <morphis@gravedo.de>
ofono: map powered/online properties of the modem interface correctly to the power status

In terms of the telephony service a ofono modem is powered when both properties (online
and powered) are true. As this means that rf functionality is enabled and that is what the
telephony services a powered device to have.

Open-webOS-DCO-1.0-Signed-off-by: Simon Busch <morphis@gravedo.de>