ndk-build: support a multiple architecture build

Fixed issue gstreamer/gst-docs#27.

This commit is to support that developers can specify a multiple values
with 'APP_ABI' of Application.mk file. The recent Adroid NDK tool provides
the multiple architecture build as well as a single architecture build.

* Reference:
   * https://developer.android.com/ndk/guides/abis?hl=en#gc

* How to evaluate with this commit
```bash
$ cd {your_work_dir}/gst-docs/examples/tutorials/android/android-tutorial-1/jni
$ cat ./Application.mk  | grep APP_ABI
APP_ABI = armeabi armeabi-v7a arm64-v8a x86 x86_64
$ ndk-build (with android-ndk-r16b)
$ tree  ../libs/
../libs/
|-- arm64-v8a
|   |-- libc++_shared.so
|   |-- libgstreamer_android.so
|   `-- libtutorial-1.so
|-- armeabi (Note: It will be deprected since android-ndk-r18b.)
|   |-- libc++_shared.so
|   |-- libgstreamer_android.so
|   `-- libtutorial-1.so
|-- armeabi-v7a
|   |-- libc++_shared.so
|   |-- libgstreamer_android.so
|   `-- libtutorial-1.so
|-- x86
|   |-- libc++_shared.so
|   |-- libgstreamer_android.so
|   `-- libtutorial-1.so
`-- x86_64
    |-- libc++_shared.so
    |-- libgstreamer_android.so
    `-- libtutorial-1.so
```

* Self assessment:
   * Local test with ndk-build: It's okay.
   * Local test with gst-docs (./android-tutorial-{1-5}/)
3 jobs for upstream-multiple-APP_ABI in 52 minutes and 29 seconds (queued for 56 minutes and 44 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #64983
manifest

00:00:19

 
  Build
passed #64984
cerbero deps fedora x86_64

00:41:32

 
  Full Builds
passed #64985
build cerbero fedora x86_64

00:10:37