Fix case compare in SkFontMgr_android_ndk

Case comparision for language tags is ASCII only, but CSS3 requires
Unicode case folding rules (without dotless I) for other name
comparisons. Use NDK ICU on Android 12 and later for full compliance and
towlower on Android 11 which handles all simple case folding.

Change-Id: Ic279937ad9e9c5da506322629e73f59961d34874
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1049516
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>