nitg: Extract exec_and_check from Android

Signed-off-by: Djomanix <johan.kayser@viacesi.fr>
nitg: Fixed the copyright notice on android_platform

Signed-off-by: Djomanix <johan.kayser@viacesi.fr>
nitg: Use template::write_to_file in android_platform

Signed-off-by: Djomanix <johan.kayser@viacesi.fr>
nitg: Added PNaCl support for Nit

Signed-off-by: Djomanix <johan.kayser@viacesi.fr>