We should be testing latest NDK release but keep using API level 21 to
ensure GLib does not start using newer APIs. We could also later add a
runner for latest API level 28 which includes iconv API in Android's
libc so we don't need GNU libiconv anymore.
This adds to the CI a cross build for Android NDK r16 API 21 (because
thats what GStreamer currently use) for arm64.
GNU iconv must be built manually into our docker image because Android
NDK doesn't seems to ship it. The latest NDK r17 API 28 has iconv.h but
iconv_open() symbol isn't found by the linker. Looks like broken NDK.
libffi also needs to be built manually because the meson subproject
doesn't support building for Android platform. It needs a recent RC
release because latest stable release is 4 years old and fails to
build.
Fixes#1385.
