Expose the empty MmTelCapabilities constructor

1) Allow for empty MmTelFeature.MmTelCapabilities to be
accessible via @SystemApi.
2) Add correct @nonnull/@nullable annotations as per API
review.

Bug: 117555643
Bug: 116117215
Test: Manual
Change-Id: Id445e4702f44528b4b1ab59129cf35803ae70053