Preserve leading "+" in phone numbers

The normalizeNumber method in PhoneNumberUtils preserves a leading "+"
but only if it's the first character in the string. Thus it gets
dropped if there is leading punctuation such as "(". This patch
preserves the leading "+" as long as it would be the first character
in the normalized phone number.

Bug: 18418335
Change-Id: Ifcb1d3dd6c2943c729f570e7f3c90d1da9ee8b4b