Fix MmTelFeatureConnection Memory Leak

In FeatureConnection, we never cleaned up the ImsFeatureStatusCallback
after it was destroyed, leaving zombie FeatureConnections around.

Bug: 156893040
Fixes: 157948894
Test: atest FrameworksTelephonyTests
Merged-In: Idcf8b558c3807a3f9f5094afee532e76ea6f02cb
Change-Id: Idcf8b558c3807a3f9f5094afee532e76ea6f02cb