ci: Add latest Android API level 28

- Split the download part into a separate script to so docker keeps that
step in cache and avoid redownloading it.

- With API level >= 28 libiconv is not needed anymore because it's part
of Android's libc.

- Generate standalone toolchains to reduce the docker image size. It's
also easier because it doesn't need to pass sysroot args.

- Use clang compiler because gcc is deprecated in this Android NDK and
will be removed in the next release.

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: registry.gitlab.gnome.org/gnome/glib/master:v4
+image: registry.gitlab.gnome.org/gnome/glib/master:v5
 
 stages:
   - build
@@ -48,6 +48,13 @@ cross-android_api21_arm64:
     - meson --cross-file=/opt/cross_file_android_arm64_21.txt -Diconv=gnu -Dinternal_pcre=true --buildtype debug _build
     - ninja -C _build
 
+cross-android_api28_arm64:
+  <<: *cross-template
+  script:
+    # FIXME: add --werror
+    - meson --cross-file=/opt/cross_file_android_arm64_28.txt -Dinternal_pcre=true --buildtype debug _build
+    - ninja -C _build
+
 cross-mingw64:
   <<: *cross-template
   script: