Mark MODIFY_PHONE_STATE permission as signatureOrSystem

This permission isn't needed right now, since there aren't actually any
public APIs that require it.  (There are a few calls in the ITelephony
interface that do, but they're all hidden.)

Since there's no good reason for 3rd party apps to declare it, let's
mark it signatureOrSystem for now.  We can bring it back -- and probably
split it apart into multiple finer-grained permissions -- once we
finally expose full telephony APIs to 3rd party apps (see bug 1043005).

Bug: 2989096
Change-Id: Idf898d5e12d648a959f622cd815e75597195aa82