ofono: do not use context path as network group identifier

ofono: do not use context path as network group identifier

The ofono plugin was using context path as network group identifier
which caused service path to end something like this:

/profile/default/cellular_244053111242822_huawei2_primarycontext1

But the problem here is that with certain modems (like my Huawei E1552)
the path will change every time the modem in replugged. This meant
that a new connman service was created everytime and I had to
enter APN everytime, instead of using the stored service which already
had APN correctly set.

Fix this by using only the last part from path, for example in this
case primarycontext1:

/profile/default/cellular_244053111242822_primarycontext1

Now with huawei modems the service id stays always the same as it should.