Fix build when using Android NDK >= r9

Android NDK tools places all generated libraries under `$PROJECT/libs` directory.
Since android-ndk-r9, at least on Ubuntu, NDK Tools places generated `*.so` under `libs/` while we expect them under `$(EXTERNAL_DIR)/libs` and `$(JNI_DIR)/libs`.
The solution is to specify libraries directory using `NDK_LIBS_OUT`.

NDK docs:
android-ndk-r9b/docs/OVERVIEW.html III.6/ Specifying custom output directories

Reports about this bug:
https://groups.google.com/d/msg/sqlcipher/cbt5ZOVMgEk/kiiWi4e8fIoJ
https://groups.google.com/d/msg/sqlcipher/WqDDsH4nUtE/EYUcSJ8G2L0J