Add the ability to remove FeatureConnection callbacks

FeatureConnection status callbacks were not being cleaned up and
were staying around indefinitely, even after the FeatureConnection
was destroyed.

Bug: 156893040
Fixes: 157948894
Test: atest FrameworksTelephonyTests
Merged-In: I28dc51b10dc230bc8f7a03b7373510cf3a29e93a
Change-Id: I28dc51b10dc230bc8f7a03b7373510cf3a29e93a