Move MmTelFeatureConnection DeathRecipient to the Main Thread

The DeathRecipient callback was being called on a binder, which
in some cases, caused the Framework to deadlock. Moved the
callback to the main thread.

Bug: 120151611
Test: install ImsTestApp test app, set as ImsService, kill app
Change-Id: I85d303a51dcc5fb5068b12a7c057f22def2473ee