Android: Correct setting ANDROID_NDK_PLATFORM

Instead of using always just the minimum SDK version
we need to differentiate between SDK and NDK version.

Fixes: QTCREATORBUG-21536
Change-Id: I2f99c9d40ab05ccd2a4b8efeb2cd0300ecf0cf3a
Reviewed-by: BogDan Vatra <bogdan@kdab.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Vikas Pachdha <vikas.pachdha@qt.io>

src/plugins/android/androidqtversion.h

@@ -55,10 +55,12 @@ public:
 
     QString description() const override;
     QString targetArch() const;
+    int mininmumNDK() const;
 protected:
     void parseMkSpec(ProFileEvaluator *) const override;
 private:
     mutable QString m_targetArch;
+    mutable int m_minNdk = -1;
 };
 
 } // namespace Internal