Call SST#setImsRegState on ImsPhone set reg state

* Previously, when Ims is deregistered through the ImsRegistrationImplBase,
TelephonyManager#setImsRegistrationState would also have to be called so
that service state tracker gracefully shutdown the radio.  The code path
from ImsRegistrationImplBase is now joined to the same logic as
TelephonyManager#setImsRegistrationState.

Test: Ims call and unit tests
Bug: 167294783
Change-Id: I2e38a8a0be880cc3d127d4fcdf12675acc2cce18