gbproxy: Use IMSI cache for PTP paging and implement DUMMY_PAGING_PS

Some code was missing to support PAGING_REJECT and DUMMY_PAGING_PS over both PTP
and signalling. This commit adds the missing pieces, notably:

* Use and route according to the IMSI cache for paging on PTP
* Ensure DUMMY_PAGING_PS is broadcast if no routing area is included

Change-Id: I7243e0d4470cb62fa6db36d26002ccd6542b5147
Related: OS#4951, OS#4472
1 file changed