Move initialization of Multiendpoint interface into getImsService().

I previously had the setup of the multiendpoint interface in ImsPhone,
however at that point ImsManager was not yet configured.
I have moved the multiendpoint interface initialization into the
ImsPhoneCallTracker#getImsService() method (similar to the ECBM interface
setup).

Bug: 27458894
Change-Id: I2b2ba9637686bb01fb38e3195e60ae151bf9adb6