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.