Deprecate SystemApi that was accidently made public in P

ImsFeature.Capabilities was accidently made public in P
due to a reference to it in another @SystemApi method.
Marking as @Deprecated in Q and then moving back to @hide
in R.

Bug: 115639348
Test: manual
Change-Id: Iab0d0c9227798b9f4e153d1c5a0a4e16353ee87c