[gen_l10n] Fix unintended use of raw string in generateString

[shihaohong]

Fixes #69345, which fails because of a change that uses raw strings as part of the code generation introduced by #69025.

This PR simply reverts the generateString logic for the gen_l10n tool, but keeps the Material/Cupertino generated localizations one intact. Since the introduced logic prefers raw strings (which the flutter/flutter linters now enforce), this is necessary for the Material and Cupertino library code generators. However, this approach fails with gen_l10n.

For example:

price: $4.45 => "r'price: $4.45'"

get String getPriceOfItem => r'price: $4.45';

^ This is okay in our cupertino and material delegate generation code because it parses raw strings with interpolated variables differently (see this example).

However, in gen_l10n, this becomes a little weird, both in the code comments:

/// Example of the code that is generated with the string above
/// is r'price: $4.45' but we really want 'price: $4.45'

and also in the delegate code:

  @override
  String price(int priceValue) {
    final intl.NumberFormat priceValueNumberFormat = intl.NumberFormat.decimalPattern(
      locale: localeName,
      
    );
    final String priceValueString = priceValueNumberFormat.format(priceValue);

    return r'$${priceValueString} SAR';
  }

instead of

  @override
  String price(int priceValue) {
    final intl.NumberFormat priceValueNumberFormat = intl.NumberFormat.decimalPattern(
      locale: localeName,
      
    );
    final String priceValueString = priceValueNumberFormat.format(priceValue);

    return '${priceValueString} SAR';
  }

Related Issues

Fixes #69345

Tests

I added the following tests:

• Fixes the failing unit test that was turned off.
• Added a new integration test for dollar signs in the arb file
• Modified an existing currency integration test which would fail if a non-breaking space character wasn't used. It is too hard to decipher and not easy to create with the keyboard, so I moved it to a language (zh) that doesn't contain the character.

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[jonahwilliams]

LGTM (but not to CI...)