Adds MMTel APIs in temporary namespace

This change adds the new MMTel APIs in a
temporary android.telephony.ims.internal
namespace so that integration work can start
from the ImsService side.

Once the APIs are finalized further and integrated
into the platform, the namespace will be moved
back to android.telephony.ims.

Tests and integration CLs to follow in
separate CLs. Anything that is not implemented
yet has a TODO.

Test: Manual
Bug: 63987047
Change-Id: Iec5e15438f6259c6edd9a308e66bebc92685c578