Prevent VVM from sending SMS in ECBM

Sending non-emergency SMS would exit ECBM mode. Visual voice mail
internally uses SMS for activation, deactivation, status check etc. So,
these actions are silently blocked when phone in in ECBM.

Bug: 177439988
Test: Sanity,
      atest com.android.internal.telephony.SmsControllerTest
Change-Id: Icfb050d8ba7daf9f4f37b51705fcab769f4c8823