ndk-build: support a multiple architecture build

Fixed issue gst-docs#27 (closed).

This commit is to support a multiple architecture build 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

$ 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 deprecated 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-i{1-5})