Fix Turkish casing on Android by deriving it from the locale name

[MsfPablo]

Fixes #106560

Root cause

TextInfo.NeedsTurkishCasing (src/libraries/System.Private.CoreLib/src/System/Globalization/TextInfo.Icu.cs) decides whether to use ChangeCaseTurkish by probing collation:

return CultureInfo.GetCultureInfo(localeName).CompareInfo.Compare("\u0131", "I", CompareOptions.IgnoreCase) == 0;

On Android the runtime loads the platform's system ICU (libicuuc.so/libicui18n.so), which does not provide the collation tailoring this probe depends on. The probe therefore returns false for tr-TR, IcuChangeCase falls through to the non-Turkish ChangeCase, and "i".ToUpper(tr-TR) yields I (U+0049) rather than İ (U+0130).

The same collation gap is already acknowledged in the test suite, which skips the 'ı' vs 'I' CurrentCultureIgnoreCase cases on Android with a pointer to this issue (src/libraries/System.Runtime/tests/System.Runtime.Tests/System/CharTests.cs:178).

Fix

Derive the flag from the locale's language subtag instead. ICU itself applies the dotted/dotless i rules to exactly the tr and az languages (ucase.cpp treats both as LOC_TURKISH), so this is equivalent to the collation probe on platforms where the probe worked, while being independent of the collation data actually shipped by the host.

Testing

Not verified on a device — I do not have an Android emulator/runtime build available here, so this is based on source analysis. Casing behaviour on other platforms should be unchanged, since tr/az are the only languages for which CLDR collates U+0131 equal to I under IgnoreCase. The disabled casing assertions referencing #106560 would be the natural way to validate this on CI; I left them untouched, as the Compare-based assertions in CharTests exercise collation rather than casing and are not fixed by this change.

Note: @murathanoktem mentioned on the issue that they were investigating. Happy to close this in favour of their PR — I am posting it because I had the root cause isolated.

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[MsfPablo]

Closing as duplicate — superseded by #132002, which includes the same fix plus the regression test.