Clean up ImsFeature status callbacks correctly in FeatureConnection

1) ImsFeatureStatusCallbacks were not being deregistered when the
MmTelFeatureConnection was removed, causing callbacks to still call
into that FeatureConnection. This caused a memory leak as well as
duplicate callbacks into ImsManager.

2) Small refactor to better decouple the abstract FeatureConnection
from the MmTelFeatureConnection

Bug: 156893040
Test: atest FrameworksTelephonyTests CtsTelephonyTestCases:ImsServiceTest
Change-Id: Iedf820f9da10088c845b34c2a08658d7204241b3