[Android] Don't attach the same thread twice to the JVM

ANativeActivity_onCreate is called by NaviteActivity from Java (which is the main thread). Thus, this thread is already attached to the JVM.
[Android] The armeabi version of SFML is now built by default (the example as well)
[Android] Forgot to dettach the SFML thread from the JVM at several places
[Android] Fixed unicode value of text entered event is always 0
Fixed some more warnings

- Updated the toolchain file to no longer show warnings for missing
prebuilt toolchains (it's perfectly fine).
- Simplified variable adjustment for x64 toolsets (still not perfect).
- Updated API versions for the example app (use latest SDK/NDK).