Revert the security exception for voip phone account registration.

In ag/30238448 I modified the security exception text; reverting it back
to what it was to hopefully prevent a CTS test failure (which I did in
ag/30350056 but for some reason still isn't sticking).  🤷

Test: Run CTS tests
Flag: com.android.server.telecom.flags.enforce_transactional_exclusivity
Fixes: 378817915
Change-Id: If56767456108b0717ea98611f68f7514c90f3991