Android, FFmpeg: Move multi-NDK logic outside install-ffmpeg-android.sh

Currently, this script handles building FFmpeg, in multiple variants,
depending on what environment variables happen to be available. This
structure makes it harder to run the script locally, as well as
maintaining it over time. This patch moves the logic for doing multiple
NDK variants into a separate script.

As a drive-by, we give both install-ffmpeg-android.sh and
install-ffmpeg-android-multi-ndk.sh named parameters.

It is now possible to run the script locally.

Pick-to: 6.12
Change-Id: Iaa4a4c8f9123830eaf88be04a8a2c845c6a62982
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>