LPP: Fix password complexity for uncased scripts

Update GetCharClass to use u_isalpha, ensuring characters from uncased
scripts (Arabic, Japanese, etc.) are correctly identified as letters.
This allows passwords using these scripts to satisfy complexity
requirements. Added regression tests for uncased scripts and Emojis.

Bug: 445625494
Test: chromeos_unittests --gtest_filter="LocalAuthFactorsComplexityCheckerTest.PasswordComplexity"
Change-Id: Ic16b60df37c0d32035ccd0f026afe22d2b8d9324
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7850081
Reviewed-by: Fahad Mansoor <fahadmansoor@google.com>
Commit-Queue: Ivan Šandrk <isandrk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1632745}