Make measureText(String, int, int) not use context

The measureText method when applied to a string should just measure
the substring, rather than treat the entire string as context. It was
less likely to cause problems than the similar issues with char
arrays, but still wrong. This patch makes the behavior consistent.

Bug: 20087437
Change-Id: I1c6e07a694b151f4fb097edae8e271805e996d06