[cross_file] fixed readAsString decoding in-memory bytes as UTF-16

[glitchfl]

XFile.fromData(utf8.encode('😀')).readAsString() hands back mojibake

The bytes branch uses String.fromCharCodes which just widens each byte into
its own UTF-16 code unit so anything outside ASCII comes out mangled and the
encoding parameter the method accepts never gets used at all. The file-backed
branch right below it passes encoding through fine, and web already does
readAsBytes().then(encoding.decode) so this is mostly just making native do
what web has been doing all along.

I made the method async while I was in there and that bit is deliberate
rather than cosmetic: encoding.decode can throw on bad input where
fromCharCodes never could and without async that would come out
synchronously instead of as a failed future, which isn't what you get from
either the web version or the _file.readAsString path.

fixes flutter/flutter#165120

AI usgae: I used antigravity (with gemini 3.7 flash)

[gemini-code-assist]

Code Review

This pull request updates the readAsString method in XFile to decode bytes using the provided encoding instead of String.fromCharCodes, fixing an issue where multi-byte characters were incorrectly decoded. It also adds corresponding unit tests for multi-byte characters, non-default encodings, and malformed data. The reviewer suggested avoiding the async keyword in readAsString to prevent unnecessary microtask delays and overhead, recommending the use of Future.sync instead.