Add and trigger onReady API for SMS over IMS.

Test: manual test that normal code path is fine since this code path is
not yet exercisable.
BUG=69846044
Merged-In: Icb15ca4aa6606fba641f6270dca5e0e06fc4466a
Change-Id: Icb15ca4aa6606fba641f6270dca5e0e06fc4466a