Fix a bug in String.charAt() simplification.

Do not pass method index as a bool flag indicating that the
HBoundsCheck originates from a String.charAt(). This was
working only thanks to the method index unlikely to be 0.

This bug was introduced in
    https://android-review.googlesource.com/321573 .

Test: Rely on TreeHugger.
Bug: 30933338
Change-Id: I2a51e478ee145d342af8cd49f9fdec7adffd77ff