Add Android CI

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.
5 jobs for android-ci in 32 minutes and 4 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #37843
fedora-meson-android_ndk_r16_api21_arm64

00:16:22

passed #37842
fedora-meson-x86_64

00:14:59

passed #37844
win32
msys2-mingw32

00:08:12

passed #37845
win32
vs2017-x64

00:05:56

 
  Coverage
passed #37846
coverage

00:01:09

76.7%