Update/clear connection proxy of RIL upon single SIM and dual SIM switch

For RIL(CommandInterface), when corresponding modem goes away
(dual-SIM to single-SIM switch), RIL should clear mRadioProxy and
mOemHookProxy. When it shows up (single-SIM to dual-SIM switch),
it should immediately connect to the RIL service.

Bug: 141388730
Test: unittest
Change-Id: I8e09c97750d0849605b1c585df9702f40a20cb16