[Android] ARM64: Split Android.mk file

[Android] ARM64: Split Android.mk file

This commit is to split the existing Android.mk file in order to maintain
(1) nnstreamer library (e.g., libnnstreamer.so) and (2) test applications
separately.

* Before: Android.mk
* After :
  - Android-nnstreamer.mk (to generate nnstreamer library)
  - Android-app.mk (to generate nnstreamer-based test application)

**Changelog**

* Version 2:
1. Added two functions to simplify duplicated statements with foreach
2. Updated the existing comment to build C/C++ source code with ndk-build

* Version 1:
1. Added Android-nnstremaer.mk
2. Added Android-app.mk

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>