Show remaining character count in SMS app

## Summary

This adds character counter below the “Send” button in SMS conversation. It
uses format XXX/Y where XXX is number of characters that can be added
without splitting the SMS into multiple messages (see article
Concatenated SMS on Wikipedia). Y is number of messages in in current
concatenated SMS. The counter is not visible when insertion of 10 7-bit
or 16-bit (depends on SMS encoding) does not create concatenated SMS.

SMS encoding is automatically guessed. 8-bit encodings are not
supported. If the message contains characters that are not supported by GSM 7-bit
encoding, counter automatically switches to UCS-2.

## Test Plan

Try entering some text that is longer than 150 characters in [GSM 03.38 encoding][1] or 60 characters in UCS-2. Number of remaining characters should be visible below the “Send” button. The character counter should show `0` at exactly 160 or 70 characters. Inserting one character should switch the counter to [Concatenated SMS][2] mode when number of messages is shown.

It should show exactly same number as SMS app in Android.

## Screenshots

These images are in APNG.

![grab.apng](/uploads/21ae23f2fa75c7aca487e61ddce94644/grab.apng)
![grab-cz.apng](/uploads/785e670a8598c5a65a4209f17e75f578/grab-cz.apng)

[1]: https://en.wikipedia.org/w/index.php?oldid=932080074#GSM_7-bit_default_alphabet_and_extension_table_of_3GPP_TS_23.038_/_GSM_03.38
[2]: https://en.wikipedia.org/w/index.php?oldid=943185255#Message_size