Remove "one shot" type of FeatureCallback

Instead of having a special case for ImsManager#getInstance,
implement it properly.

Test: atest CtsTelephonyTestCases

Merged-In: I2e986fb2a1086043a25b11c71c005481cea74196
Change-Id: I2e986fb2a1086043a25b11c71c005481cea74196