NetworkManagementService: Change exception type thrown on IPv6

Changed the exception type thrown from IllegalStateException to
RemoteException whenever the IPv6 tethering rules fail to apply. The
IllegalStateException is not caught by the caller causing the JVM
to crash. By throwing RemoteException, the caller will catch and
print and error message instead of crashing.

CRs-Fixed: 662171
Change-Id: I92fadd43479a330f09bb20f699a4b9852123ab68