Fixed several issues

- Cmake errored out due to the ANDROID_NDK_* variables being used before being set/detected for the first time.
- Fixed one warning regarding one string replace in CMake.
- Fixed warnings when compiling SFML-Activity.

Theres are a few more issue I didn't touch, since I don't know how you'd like to handle these: SDK/NDK versions and dependencies:

* By default, only the *armeabi-v7a* version of SFML is built, but the Android example wants to compile all available architectures (which will fail due to SFML's *armeabi* version missing).
* There's no valid *all* target architecture for SFML.
* There are several mixed/mismatched version numbers in the example project, e.g. the version numbers in the *AndroidManifest.xml, project.properties,* and *Application.mk.*