Only send US numbers over GVoice

Google Voice only allows sending SMS to US numbers. On non-US phones,
it makes sense to send normal SMSes through the normal cellular channel,
while sending US SMSes via GVoice. This is, at least, the most common
work flow for non-US phones that use GVoice.

== Examples ==

A US number:
    [international format] +1 303 555 1212
    [local format]         303 555 1212
A French Number:
    [international format] +33 6 99 99 99 99
    [local format]         06 99 99 99 99

When I am in the United States:
   - Sending to "+1 303 555 1212" should occur over Google Voice, as it is a US number.
   - Sending to "303 555 1212" should occur over Google Voice, as locally it is a US number.
   - Sending to "+33 6 99 99 99 99" should occur over CELLULAR as it is an international number.
   - Sending to "06 99 99 99 99" should occur over Google Voice, as locally it is a US number (even though it's invalid).
When I am in France:
   - Sending to "+1 303 555 1212" should occur over Google Voice, as it is a US number.
   - Sending to "303 555 1212" should occur over CELLULAR, as locally it is a French number (even though it's invalid).
   - Sending to "+33 6 99 99 99 99" should occur over CELLULAR as it is a French number.
   - Sending to "06 99 99 99 99" should occur over CELLULAR, as locally it is a French number.

This patch is untested.

Change-Id: I0904130c52031bf2bc071e5d1b513c506978ccfc