Enable ims test mode on verizon

Added a new property 'persist.dbg.allow_ims_off'. When it's set,
CarrierConfig KEY_CARRIER_ALLOW_TURNOFF_IMS_BOOL is ignored.
Hence ims can be turned off.

Bug: 28861107
Change-Id: Ieed8f782b6e222131e64ee8755a48acc1100e520