API Review: Make ImsReasonInfo in ImsMmTelManager @NonNull

1) If the vendor does not specify an ImsReasonInfo, create an
unspecified ImsReasonInfo instead of passing null back.
2) Move onUnregistered to @NonNull

Fixes: 150008589
Test: atest CtsTelephonyTestCases:ImsServiceTest
Change-Id: Idd0f2216bdd55011cb16d268b61cb652ca11bd41