Fix unicode-decoding mistake in isInGsmAlphabet method

Summary

Correct unicode-decoding mistake when trying to determine if the ö character is part of the GSM alphabet.

0xf5 is the ò character, while 0xf6 is the ö character.

BUG: 445776

Test Plan

Before:

Writing an SMS with the ö would incorrectly report a huge number of character used since it would not be able to use GSM encoding:

After:

Writing an SMS with the ö character should count characters as 1:1 since we can use the GSM alphabet:

This matches the count of the same message using Google Messages on Android, so it's probably correct.